Onix

As Onix grows, the stone portions of its snake body harden to become sooty black and diamond-like. In "Showdown in Pewter City," Ash finds himself face-to-face with his first Pokemon gym leader — Brock. Pikachu powers up and stands ready to do battle, but Brock lets loose the monstrous Onix who makes short work of it. After some intense and extremely weird training exercises, Pikachu battles again and topples the giant Onix.

First appearance: Episode 5—"Showdown in Pewter City"