A post of mine from Page 180 of this very thread:
Actually I'm just going to summarize what the whole post says:
Onix stats were probably higher originally.
However to make the first battle with Brock more awe-striking they decided to give him Onix but because of that needed to weaken it.
Onix stats probably resembled Steelixe's originally. High defense, decent HP and Attack, and low Special and Speed. Maybe instead of 35/45/160/30/70//340 was 70/70/140/30/30//340. (Remember in Gen I the Special Attack and Special Defense stat was all one "Special" stat).
Now Bulbasaur and Squirtle have no problem getting past Onix but Charmander is. So how about lowering is HP and Attack so it's not that long of a time for Charmander to whittle down its HP while taking not that much damage that can't be healed every so often with a Potion. However there needs to be some challenge so let's just make it faster. Also made Defense a bit higher so teach the player that its sometimes better to focus on attacking with a different category if one category isn't doing a lot of damage. While Ember is resisted, Onix's low Special stat means it would be doing more damage with it anyway than with any Physical attacks Charmander would know like Scratch.