Koffing

Because it stores several kinds of toxic gases in its body, Koffing is prone to exploding without warning. In Ash's first adventure, "Pokemon Emergency," Pikachu is rushed to the Viridian City Pokemon Center to be treated for severe exhaustion. When the evil Team Rocket show up, the pair send Ekans and the toxic Koffing to destroy the center. The barely recovered Pikachu and other Pokemon from the center join forces to defeat the Pokemon-snatching villains.

First appearance: Episode 2—"Pokemon Emergency"