Aerodactyl
Availability: Late-game (revive from Old Amber at Poké'mon Lab in Cinnabar Island, level 30).
Typing: Rock / Flying only provides it with a type advantage against Bruno's Fighting-types and a resistance to Blaine, but worsens the matchup against Lorelei. Rock is also useless offensively, as Aerodactyl learns no Rock-type moves.
Stats: Aerodactyl's Speed is incredibly high and grants it a 25.39% chance of landing a critical hit. Its 105 Attack is also relatively high, though Aerodactyl is not very bulky.
Movepool: Aerodactyl's STAB moves consist of Wing Attack and Fly and later Sky Attack through TMs. Its only other forms of coverage include Bite at level 38, Fire Blast and Double-Edge through TMs, and Hyper Beam at level 54.
Major Battles: Aerodactyl performs well against Sabrina and Blaine. At the Pokémon League, it can OHKO frailer Pokémon like Agatha's Ghost-types and Lance's Dragonair with Sky Attack. The other matchups tend to be bad for Aerodactyl due to its limited movepool and low bulk.
Additional Comments: Aerodactyl comes rather late and is not particularly useful in most of the remaining fights. Old Amber can be obtained in the backroom of the Pewter Museum of Science, though you need Cut to access the room.
Caterpie
Availability: Early-game (Virdian Forest, level 3 (R), 3-5 (B), or 3-6 (Y)).
Typing: Bug / Flying is only useful for resisting Erika. It gives Butterfree a weakness to RB Koga, Lt. Surge, and Blaine and is useless offensively, as Butterfree doesn't learn STAB moves at all.
Stats: Butterfree's stats are very high for the earlier portions of the game, though as the game goes on, they will start to fall off.
Movepool: Butterfree learns Confusion and Sleep Powder at levels 12 and 17 in RB and levels 10 and 15 in Yellow. Confusion upgrades to Psybeam at level 32 (RB) or 34 (Y) or to Psychic through TMs. Mega Drain through TMs provides coverage against Rock-types.
Major Battles: With a Psychic move, it can beat Brock, Koga, Erika, and Bruno with little difficulty. Brock is especially notable as in comparison to most other mons, Butterfree can hit hard with Confusion and play around Bide with Harden. Against the rest, it typically has to put them to sleep with Sleep Powder and take a lot of time KOing them with Psybeam / Psychic.
Additional Comments: Butterfree is good for few fights, but its low stats force it to put most opponents to sleep and slowly take them out, which generally takes a lot of time to pull off.
Chansey
Availability: Mid-game (Safari Zone, Center Area (hub), Center Area (east), Area 2)
Typing: Normal typing has neutral matchups everywhere save for Agatha and Bruno's Fighting types (which Chansey would use Psychic for).
Stats: 250 base HP and 105 Special make Chansey the ultimate Special tank, though its Speed is low and its physical stats are lacking.
Movepool: Chansey's natural learnset isn't really worth mentioning aside from Sing and Light Screen at levels 24 and 48. However, Chansey has an expansive TM movepool of great Special options like Ice Beam, Thunderbolt, and Psychic, with options like Reflect + Softboiled and Thunder Wave also available for utility if desired.
Major Battles: It is recommended to give Chansey Psychic and Ice Beam to succeed against Koga, Giovanni (watch out for physical moves though), Bruno, Agatha, and Lance. Thunderbolt can help for Lorelei as well.
Additional Comments: Chansey's 1% or 4% encounter rate, low catch rate, and high reliance on TMs prevent it from being tiered higher, though its Fast growth rate keeps it relevant. Chansey is not a bad Pokemon, but it simply requires too much investment needed to perform effectively.
Cubone
Availability: Mid-game (Poémon Tower, level 20 (all games) or 22 (RB)).
Typing: Ground is useful for RB Koga, Blaine, and Agatha, but bad for Erika and Lorelei.
Stats: Marowak's stats outside of 110 Defense are not particularly high; it is rather slow and its 80 Attack isn't that notable. Its low Special also prevents it from using the special TMs it can learn.
Movepool: Although Cubone starts with Bone Club, it vastly prefers Dig or Earthquake through TMs due to their higher power and accuracy. It also learns Bonemerang at level 48 as Marowak as a replacement, though it comes rather late. As it cannot use special TMs like BubbleBeam, Ice Beam, and Fire Blast due to its low Special, it is limited only to Strength for physical coverage.
Major Battles: Marowak is typically useful only against Ground-weak opponents like RB Koga, Blaine, and Agatha and generally struggles to OHKO the other opponents due to its unimpressive Attack.
Additional Comments: Despite learning Ground moves by level, they are either weak or come late, thus Cubone is still reliant on the Ground TMs to be useful. Furthermore, even with them, Cubone is generally limited in its contributions when it doesn't have a type advantage. In Yellow, Cubone is available from the fifth floor of Pokemon Tower onwards.
Dratini
Availability: Mid-game (buy at Rocket Game Corner for 2800 (R) or 4600 (B) coins; Safari Zone (Central Area (hub), with Super Rod) (Y)).
Typing: Dragon gives it a bad matchup against Lorelei, though it also resists Erika. It is useless offensively, since Dragon STAB moves do not exist.
Stats: Dragonair's stats are not particularly high, with Dratini's stats being even worse. Dragonite's stats are very high, but, due to late evolution, it is unlikely that the line will get to use them.
Movepool: It learns a variety of TMs, such as Surf, Body Slam, Ice Beam, Blizzard, Thunderbolt, and Fire Blast. It can also utilize the combination of Wrap and Thunder Wave or Agility to slowly weaken the opponent.
Major Battles: With Surf, Dratini proves useful for Blaine, Giovanni, and Bruno. Ice Beam can also improve the matchup against Erika and Lance. Thunder Wave + Wrap can be used to put opponents in range for one of its moves to KO them.
Additional Comments: Dratini is not easy to obtain and performs well only in few matchups, with contested TMs being needed to perform better in more. It also levels up slowly. In Yellow, you can reel in a level 15 Dragonair (10% encounter rate) in the Safari Zone's Central Area (hub).
Ekans (R)
Availability: Early-game (Route 4, level 6, 8, 10, or 12) (R).
Typing: Poison only gives it a resistances Erika and Koga and a weakness to Sabrina. Arbok doesn't learn any STAB moves, making its type useless.
Stats: Arbok's Attack and Speed are the only mildly impressive stats it has, though even they fall off later on. It is also fairly weak as Ekans.
Movepool: Ekans relies on Leer, Wrap, and Poison Sting until it learns Bite at level 17. Arbok depends on the Dig or Earthquake TM (the former being preferred early on) to perform well. Other useful moves include Glare at level 27 and Mega Drain, Rock Slide, and Strength through TMs.
Major Battles: If it knows Dig (or Earthquake later on), it will perform well against Lt. Surge, Koga, Blaine, and Agatha. It also performs well against Giovanni (with Mega Drain), Lorelei (with Rock Slide) and Lance's Dragon-types, because they will always use Barrier or Agility against it.
Additional Comments: Arbok is held back by its bad Ekans period, especially before it learns Bite. Furthermore, it's reliant on super effective hits to maintain viability later on, so its performance worsens greatly if it's not taught good TMs.
Electabuzz (R)
Availability: Late-game (Power Plant, level 33 or 36).
Typing: Electric gives it a good matchup against Lorelei, although it's useless against Giovanni.
Stats: Outside of decent offenses and 105 Speed (which gives it a 20.50% chance of landing a critical hit) its bulk is unimpressive.
Movepool: Although Electabuzz learns Thunderpunch at level 42, it is still better to give it the Thunderbolt TM, as it otherwise has to rely on Thundershock until then and because Thunderpunch misses on many KOs that Thunderbolt doesn't. Outside of those, Electabuzz can also be taught Psychic, Strength, and Submission through TMs for coverage.
Major Battles: Electabuzz is useful against Lorelei, Bruno, and Lance's Gyarados and Aerodactyl. It will generally do fine against nonspecially bulky like Gengar or Alakazam, though the latter can be handled with Strength.
Additional Comments: Electabuzz comes rather late and is not powerful enough to be useful for what is left of the game, though its high chance of landing a critical hit can compensate for its unremarkable offensive stats.
Farfetch'd (Y)
Availability: Mid-game (Route 11, level 26 or 31).
Typing: Normal / Flying gives it a good matchup against Erika, Koga, and Bruno's Fighting-types, but worsens the matchup against Lorelei.
Stats: Farfetch'd stats are rather low and do not hold it for long, thus it has to rely on setup to compensate for them. Its Speed stat gives its Slash around 93% chance to land a critical hit.
Movepool: Farfetch'd greatly appreciates the Body Slam TM in order to hit hard immediately, with Fury Attack at level 15, Double-Edge through TMs, and Slash at level 39 being other alternatives. Swords Dance and Agility at levels 23 and 31 help it sweep, with the former combining well with all STAB moves bar Slash. Its only Flying-type option is Fly through HM.
Major Battles: Farfetch'd generally needs to set up with Swords Dance and potentially Agility in order to sweep any team. This is unreliable at the Pokémon League, as its low bulk makes it hard to set up consistently. Farfetch'd can also just pick off frail targets with Slash critical hits.
Additional Comments: Farfetch'd has low stats and therefore is unable to perform well anywhere without having to set up. Furthermore, even with setup, it is an unreliable choice against the Elite Four, which further limits its usefulness.
Grimer
Availability: Late-game (Pokémon Mansion (1F), level 30 (R), 30 or 34 (B) or 26, 29, 32, or 35 (Y)).
Typing: Poison is not useful for what is left of the game and gives it a weakness against Sabrina and Giovanni.
Stats: Muk has a high HP and Attack, though it's slow and its Special prevents it from using special TMs effectively.
Movepool: Grimer's only STAB move is Sludge at level 37. TM-wise, it can be taught Mega Drain, Thunderbolt, and Fire Blast.
Major Battles: Muk is able to take on physically frail targets with Sludge and on threats it can hit super effectively with TMs. In RB, it is useful against Lance, as his Dragon-types will always use Barrier or Agility against it.
Additional Comments: Grimer comes late and is not particularly useful for what is left of the game. You can catch it directly as a Muk at a higher level with the help of Repels, though its level and rarity vary per game and floor. Grimer is more common in Blue and Yellow than in Red.
Growlithe (Y)
Availability: Late-game (Pokémon Mansion, level 26, 30, 34, or 38 (first floor)).
Typing: Fire does not give Growlithe any advantages at this point of the game and makes it weak to Giovanni. It is also resisted by Lance.
Stats: Arcanine's stats are all around good, most notably having a high Attack and being relatively fast.
Movepool: The line relies entirely on Ember and TMs, as the only useful level-up move Growlithe learns, Flamethrower, comes at level 50. Arcanine's best options from TMs are Body Slam, Double-Edge, Dig, and Fire Blast.
Major Battles: Arcanine can perform well against Blaine and Agatha with Dig and also performs well against Sabrina. It can also pick off kills at the Elite Four with Fire Blast.
Additional Comments: Growlithe comes late and the only way for it to be any good is if you kept the Dig TM for that long to give it to Arcanine. Evolve with Fire Stone immediately, as waiting for Flamethrower at level 50 is not worth it.
Hitmonchan
Availability: Mid-game (Fighting Dojo in Saffron City, Level 30).
Typing: Fighting type allows Hitmonlee to be useful against Lorelei, but is questionable against Sabrina and is not useful for Koga and Agatha.
Stats: Hitmonchan has good Attack, but its Speed is not very high and its bulk (especially on the special side) is very bad.
Movepool: Hitmonchan's most optimal moveset comes from TMs. Its only STAB move is Submission, with Strength acting as coverage. Hitmonchan learns Fire Punch, Ice Punch, and Thunderpunch at levels 33, 38, and 43, but its low Special prevents it from using them effectively.
Major Battles: Hitmonchan struggles to sweep any fight outside of the first Giovanni battle. It generally contributes by OHKOing frail targets with STAB Submission, though it's also useful against Lorelei.
Additional Comments: Hitmonchan struggles to perform well against most fights due to its bad bulk and poor movepool. Its only positive traits are that it comes at a high level, its good Lorelei matchup, and its ability to OHKO some Pokémon easily.
Koffing
Availability: Late-game (Pokémon Mansion (1F), level 30 (RB) or 32 (B)) (RB).
Typing: Poison is not useful for what is left of the game and gives it a weakness against Sabrina and Giovanni.
Stats: Weezing is rather slow, though it is rather bulky, particularly on the physical side. Its 85 Special allows it to hit relatively hard.
Movepool: Koffing learns Sludge at level 32 and greatly appreciates Thunderbolt and Fire Blast through TMs. Selfdestruct and Explosion at levels 43 and 53 allow it to weaken difficult opponents later on.
Major Battles: Koffing performs well against Lorelei and Bruno's Fighting-types with Thunderbolt. Lance's Aerodactyl and Gyarados are handled by Thunderbolt, while his Dragon-types only use Agility or Barrier against it, letting Weezing win easily.
Additional Comments: Koffing comes late and is reliant on TMs to be useful. You can also catch it directly as a Weezing with the help of Repels, though its level and rarity vary per game and floor. Note that Koffing is more common in Red than in Blue.
Lickitung (RB)
Availability: Mid-game (trade a Slowbro on Route 18 (RB)).
Typing: Normal is neutral across the whole game, with only Agatha's Ghost-types being immune to it and with Bruno's Fighting-type moves hitting it super effectively.
Stats: Lickitung has below-average stats; it is slow, doesn't take hits well, and generally doesn't hit hard unless it has Swords Dance boosts or is hitting super effectively.
Movepool: Entirely TM-based. Lickitung learns Strength and Earthquake, which can be combined with Swords Dance. It can also be taught Surf for HM utility and to hit opponents like Rhyhorn hard.
Major Battles: In most fights, the only way for it to perform well is to set up Swords Dance and use an appropriate move. Even then, it will often require healing due to below-average bulk and will be unable to sweep tougher opponents like Lorelei and Lance regardless.
Additional Comments: Despite the boosted experience, Lickitung has difficulties performing well in most major fights due to its below-average stats. In Yellow, Lickitung can only be obtained in Cerulean Cave, making it unavailable for the main game there.
Magmar
Availability: Late-game (Pokémon Mansion (3F), level 34) (B).
Typing: At this point of the game, Fire does not provide it with many advantages, with Magmar losing to Lorelei and being resisted by Lance.
Stats: Magmar's stats are largely unimpressive, with its 95 / 85 offenses not helping it for what is left of the game. However, it can outspeed most opponents reasonably with its 93 Speed.
Movepool: Magmar's Ember upgrades to Fire Punch at level 43, with Flamethrower coming at level 55 if you can reach that level. Fire Blast through TMs also provides more power, while Strength, Psychic, and Submission through TMs provide coverage.
Major Battles: Magmar generally doesn't sweep any of the remaining fights, though it can contribute to Sabrina with Strength and handle the end-game with a powerful Fire Blast.
Additional Comments: Magmar comes late and is not very useful for what is left of the game, despite good STAB moves in its learnset.
Moltres
Availability: End-game (Victory Road (2F), level 50, static encounter).
Typing: Fire / Flying provides it with a neutral matchup against whatever is left of the game save for Lorelei, who Moltres is weak to.
Stats: Moltres has an incredibly high Special and the other stats aren't too bad either. However, they are held back by a lack of EVs.
Movepool: Moltres mainly uses Fire Blast and Sky Attack through TMs, with Fire Spin slowly weakening bulky Pokémon.
Major Battles: Moltres can take on most Pokémon at the Pokémon League by hitting them with an appropriate STAB move. However, it will need Ethers and Elixirs to consistently take out more of them.
Additional Comments: While Moltres is very powerful, it doesn't contribute much due to coming extremely late.
Onix
Availability: Mid-game (Rock Tunnel (B1F), level 13 or 17 (RB) or level 14, 18, and 22 (Y)).
Typing: Rock / Ground gives Onix a bad matchup against Erika and Lorelei and a type advantage against Koga, Blaine, and Agatha.
Stats: Onix has an incredibly low Attack, which means that, later on, even super effective moves won't be able to OHKO opponents. Furthermore, its high Defense is not very useful due to its low HP. It is also frail specially.
Movepool: Onix greatly appreciates Dig or Earthquake (preferably the former due to its earlier acquisition) and Rock Slide through TMs. Its only other Rock-type move is Rock Throw at level 19. Bind at level 15 can be used to slowly weaken opponents.
Major Battles: Onix is useful for Koga and Blaine as well as Agatha. It can also defeat Lt. Surge if you obtain it before fighting him. However, don't expect Onix to OHKO its opponents in the later portions of the game.
Additional Comments: Due to its terrible stats, Onix is useful only in few matchups and needs the Dig TM to become usable immediately.
Paras
Availability: Early-game (Mt. Moon, levels 8-12 (RB), 9-13 (Y))
Typing: Bug / Grass is an unfortunate type, only really helping against Misty, Sabrina, and Lorelei, but providing it with three 4x weaknesses. Stats: Parasect has well-rounded stats everywhere except for 30 base Speed, which only enables a 49% critical hit chance with Slash.
Movepool: Paras joins with Scratch, which upgrades to Body Slam through TMs and later Slash at level 39, and learns Stun Spore and Leech Life at levels 13 and 20 and Dig through TMs. Swords Dance through TMs combines well with Body Slam and Dig, while Growth at level 41 can combine with the Mega Drain TM. Spore at level 30 allows Paras to safely set up against the opponent.
Major Battles: Paras has good match-ups against Misty and Erika with Leech Life. If given the Dig TM, it is also excellent against Lt. Surge and can face Agatha. If given the Mega Drain TM, it can also target Giovanni and Lorelei. The Swords Dance TM improves every match-up significantly, although Blaine and the rival still give issues.
Additional Comments: Spore is Paras's selling point, and it helps Paras to get free turns, but it doesn't work well with Paras's low Speed. Ultimately, Paras requires multiple specific TMs to reach its maximum potential. In Yellow, you can also trade a Tangela on Route 18 to get a Parasect, though that Parasect misses out on its best match-ups.
Pidgey
Availability: Early-game (Route 1, level 2-5 (RB) or 2-4 (Y)).
Typing: Normal / Flying gives it a good matchup against Erika, Yellow Koga, and Bruno's Fighting-types. However, it is bad against Brock, Lt. Surge, and Lorelei, as well as against any Rock-type Pokémon.
Stats: Pidgey and Pidgeotto's stats are not very good and you are stuck with them for a while due to Pidgeotto's late evolution. Only after it evolves into Pidgeot do the stats become acceptable, though they fall off later on anyways.
Movepool: Pidgey's Gust and Quick Attack upgrade to Double-Edge through TMs. For a Flying-type move, it can use Fly through HM and later Sky Attack through TMs. It can also be taught support TMs like Reflect and Toxic, as the line doesn't get anything else that is useful.
Major Battles: The line beats Erika, Yellow Koga, and Bruno's Fighting-types just with a Flying STAB move. Outside of those, the line generally contributes by KOing frail opponents, though it can OHKO some threats at the Pokémon League with Sky Attack.
Additional Comments: Despite coming early, Pidgey is limited in its contributions due to its lackluster stats, typing, and movepool. You can also catch it directly as a Pidgeotto in Yellow's Viridian Forest, but it's at a 1% encounter rate, so it's not worth it.
Pinsir
Availability: Mid-game (buy at Rocket Game Corner for 2500 (B) or 6500 (Y) coins; Safari Zone (Central Area (hub)) at level 23 (B) or Area 3 at level 25 (Y)).
Typing: Bug is useless for Pinsir offensively, as it learns no Bug-type moves. Defensively, it gives Pinsir a weakness to RB Koga and Blaine as well as a resistance to Giovanni and Bruno's Fighting-types. Its Speed also makes its Slash a guaranteed critical hit.
Stats: Pinsir has one of the highest Attack stats along with a good Defense and Speed, though its other stats are bad.
Movepool: Pinsir's only viable offensive moves are Strength and Submission through TMs and Slash at level 49. It can combine Strength and Submission with Swords Dance through TMs to sweep opponents. In Yellow, it also learns Bind at level 21 for gradual trapping damge.
Major Battles: Pinsir generally performs well by setting up Swords Dance and then using an appropriate move. This strategy is ineffective against Agatha's Ghost-types, though Pinsir can damage them with Seismic Toss.
Additional Comments: Pinsir's shallow movepool forces it to rely on setup to sweep any fight. Furthermore, it levels up slowly and is a hassle to obtain regardless of the game or method.
Pikachu (Y)
Availability: Early-game (Pallet Town, starter).
Typing: Pure Electric type makes it good for Misty and Lorelei while being weak to Giovanni's Ground types and resisted by Erika.
Stats: Due to not being able to evolve in Yellow, Pikachu's below-60 offenses and paper-thin defenses fall off quickly. Its Speed of 90 gives it a 17.59% chance to land a critical hit though.
Movepool: Thundershock, Thunder Wave, and Quick Attack get Pikachu by until Thunderbolt at level 26. Body Slam via TMs is an upgrade to Quick Attack, but you'll mostly be using Thunderbolt. Light Screen at level 50 can help at the Pokemon League against Lorelei. Seismic Toss through TM can help for Electric and Normal-resistant foes.
Major Battles: Good for Misty and Lorelei, but by the time you reach Koga and onward, Pikachu's stats will be too low to do much to evolved foes.
Additional Comments: Despite learning Thunderbolt naturally, Pikachu's slightly buffed moveset from RB simply isn't enough to compensate for Pikachu's low stats lategame, and it is by far one of the worst starters of all time.
Ponyta
Availability: Late-game (Pokémon Mansion, level 28, 30, 32, or 34) (RB); Mid-game (Route 17, level 28, 30, or 32) (Y).
Typing: Fire only gives Ponyta an advantage against Erika and Koga in Yellow, while in RB, it doesn't provide it with any good matchups.
Stats: Rapidash is fast and has good Attack along with an okay Special, though its other stats are bad.
Movepool: Ponyta's STAB options are Ember, Fire Spin at level 39, and Fire Blast through TMs. Outside of that, its only forms of coverage are Body Slam and Double-Edge via TMs.
Major Battles: In Yellow, it can contribute against Erika with Ember and Koga with Fire Spin. In both games, it is useful against Sabrina and is generally good for KOing frail targets or hitting opponents hard with Fire Blast later on.
Additional Comments: Ponyta is held back greatly by its useless typing and shallow movepool. While it comes earlier in Yellow, it doesn't perform well enough to distinguish itself from its Red and Blue variant tier-wise.
Psyduck (Y)
Availability: Late-game (Route 6, level 15 (Psyduck) or 15 or 20 (Golduck)) (Surfing).
Typing: Water allows Psyduck to perform well against Blaine and Giovanni, but doesn't provide it with any advantages elsewhere.
Stats: Golduck's stats are good, but not very notable, with he highest one being its 95 Special.
Movepool: Golduck relies on Surf and Ice Beam or Blizzard through TMs. It can be taught Dig and Seismic Toss through TMs for coverage.
Major Battles: Golduck beats Blaine, Giovanni, and Bruno with no problems. With an Ice TM, it can handle Lance. It can also potentially contribute against Agatha if it knows Dig.
Additional Comments: Psyduck comes late and underleveled making it inferior to most other Water-types, despite comparable matchups.
Scyther
Availability: Mid-game (buy at Rocket Game Corner for 5500 (R) or 6500 (Y) coins; Safari Zone (Central Area (hub)) at level 23 (B) or Area 2 at level 25 (Y)).
Typing: Bug / Flying is useless offensively due to Scyther learning no Bug moves and Wing Attack (only learned in Yellow) being unhelpful. Defensively, it gives it an advantage against Erika, Giovanni, and Bruno's Fighting-types as well as a weakness to Koga and Blaine.
Stats: Scyther is strong and fast and always lands critical hits with Slash, though it's not very bulky.
Movepool: Scyther's most optimal movepool consists of Slash at level 29, Double-Edge through TMs, and Swords Dance at level 35. In Yellow, it also learns Wing Attack at level 50.
Major Battles: Scyther only performs well by setting up Swords Dance and then using Double-Edge, though it will have to heal sometimes due to recoil. It can also pick off frail targets with Slash, though it's useless against Agatha unless it knows Wing Attack.
Additional Comments: Scyther has a shallow movepool, levels up slowly, and is a hassle to get no matter which method you go for.
Slowpoke (Y)
Availability: Late-game (Seafoam Islands (1F), level 28 or 30) (walking).
Typing: Water makes it useful against Blaine and Giovanni, while Psychic gives it a type advantage against Bruno, and Agatha.
Stats: Slowbro is a bulky Pokémon that is very slow and generally needs Amnesia boosts to hit hard with its 80 Special.
Movepool: Slowbro's offensive options are Surf, Psychic, and Ice Beam or Blizzard through TMs. Amnesia at level 44 allows it to sweep opponents that are not hit super effectively by its moves.
Major Battles: Slowbro handles Blaine, Giovanni, and Bruno with little trouble. With Psychic, it can also help against Agatha if it doesn't get crippled by status moves and can help against Lance with an Ice TM. Amnesia boosts allow it to contribute elsewhere.
Additional Comments: Slowpoke comes underleveled and is hard to train due to its slow growth rate. It also needs Amnesia boosts to perform well in many matchups. You can catch it as a Slowbro directly, but it's at 1% encounter rate and Repel tricking doesn't work here.
Tangela
Availability: Cinnabar Island (RB), in-game trade for a Venonat); Mid-game (Safari Zone, levels 22-27) (Center Area and Area 3) (Y).
Typing: Grass is useful against Lorelei and Giovanni, but lackluster in other spots.
Stats: Tangela has great Defense and good Special, but low Attack and lacking Speed. This can be mitigated with the trade bonus in Red and Blue.
Movepool: Tangela's best STAB moves are Solar Beam and Mega Drain through TMs, which can be combined with Growth at level 48. It can also be taught Swords Dance and Body Slam or Double-Edge through TMs. Sleep Powder at level 39 helps Tangela set up and sweep most opponents.
Major Battles: Tangela works well against Giovanni and Lorelei thanks to Growth. With the Swords Dance TM, Tangela's other match-ups improve significantly. Agatha is a notoriously bad match-up without gimmicks like Mimic Dream Eater.
Additional Comments: Tangela can contribute to the end-game thanks to Sleep Powder, but is all around inefficient, as it requires multiple Swords Dance or Growth boosts to sweep.
Venonat
Availability: Mid-game (Safari Zone (Area 2), level 32, caught as Venomoth) (RB); Early-game (Route 24, level 13 or 16) (Y).
Typing: Bug / Poison gives it a type advantage against Erika and RB Koga while resisting Bruno's Fighting-type moves. However, it leaves Venomoth with weaknesses to Sabrina and Blaine, though it also hits the former super effectively.
Stats: It is very weak as Venonat and Venomoth's stats are not very high either, with its highest stats being 90 Special and Speed.
Movepool: In Yellow. Venonat has to manage with Tackle, Confusion at level 11, and Leech Life at level 27 until it evolves. It can also be taught Psychic and Mega Drain through TMs, with Psybeam at level 38 being an alternative for the former. Sleep Powder allows it to cripple an opponent that Venomoth would struggle against otherwise.
Major Battles: Venomoth is good for Erika with Leech Life, while Koga and Bruno are defeated with Psybeam or Psychic. It can contribute against Giovanni with Mega Drain. The other opponents need to be put to sleep, as they can take its attacks easily and wear it down in return.
Additional Comments: Venomoth is not very powerful and relies on sleep in order to beat anything that it doesn't hit super effectively. While Venonat comes earlier in Yellow, its inability to be useful for the earlier fights counterbalances the earlier arrival.
Voltorb (Y)
Availability: Late-game (Power Plant, level 43, caught as an Electrode from fake item balls).
Typing: Electric gives Voltorb a better matchup against Lorelei and a neutral matchup everywhere else bar Giovanni.
Stats: Electrode is the fastest Pokémon in the entire game and has a 27.34% chance of landing a critical hit. Its other stats are not as high, with its Special reaching 80.
Movepool: The line is heavily reliant on the Thunderbolt TM for its best performance. Outside of that, it doesn't have much to go with, other than Selfdestruct and Explosion at levels 27 (as Voltorb) and 50 (as Electrode), though it generally has to weaken the target first to KO it. Support moves like Thunder Wave and Toxic can be taught through TMs.
Major Battles: Electrode performs well against Lorelei and Bruno's Fighting-types and can KO Lance's Gyarados and Aerodactyl. Outside of that, it can generally beat neutral targets that are not too bulky specially, though it is virtually useless against anything that resists Electric.
Additional Comments: Voltorb comes rather late and, due to its unimpressive Special, is not very useful for what is left of the game, though its high chance of landing a critical hit allows it to maintain some viability against bulkier threats.
Zubat
Availability: Early-game (Mt. Moon, levels 6-11).
Typing: Flying / Poison is barely useful offensively, as Golbat learns only one (unhelpful) STAB move. It provides a resistance to Erika and Koga and a weakness to Lt. Surge, Sabrina, and Lorelei.
Stats: Golbat's stats are all around fine for most parts of the game, but fall off at the Pokémon League. The highest one is Speed, which allows it to outspeed the majority of the opponents.
Movepool: Zubat is stuck with Leech Life until it learns Bite at level 15. The only STAB move it learns is Wing Attack at level 32 as a Golbat. TM-wise, Mega Drain makes it useful for KOing threats like Onix and Rhyhorn, while Double-Edge is a stronger alternative to Bite.
Major Battles: Zubat can beat Misty, Erika, and Bruno with no problems. It can also be useful against Yellow Koga, although it should watch out for Psychic. In the other fights, Golbat generally struggles to perform well due to its low-power moves and typing.
Additional Comments: Zubat is a good Poémon for few key fights, but tends to be useless elsewhere.