Pikachu beat Onix, even back when he was a n00b at the first gym.
Although being chained to something could pose a problem as well.
Well, sort of. He used the sprinklers, and destroyed Brock's gym. Then Brock came by, said "you didn't win, but here's your badge anyway", and then left his father to care for his 86 siblings while he went and had fun. Wasn't really a straightfoward victory.
Since then, I don't think Onix ever lost.
Onix would just have the size on Chain Chomp.
This is why I love DTF. The fact that other people besides myself remember is in such great detail.
I really feel like watching the series again.
It's been many years, but the classic Pokemon could easily stand the test of time.
What pissed me off is how Ash gave away so many Pokemon!
He gave away Pidgeot, Primeape, Butterfree, Charizard, Squirtle, and tried to let Pikachu go several times. Plus, the very same episode that Togepi finally evolves into Togetic, Misty gives it away to be the Queen of the Togepi.