It made its first appearance in the episode Gymbaliar!. Here Croagunk worked with Team Rocket, for a short time, in a fake gym they had set up, defeating trainers while Team Rocket took the defeated Pokémon from them claiming to be able to train them. Toward the end of the episode Croagunk seems to follow Brock and willingly becomes his Pokémon.
Croagunk and its evolved form are the only known Pokémon to have a Poison/Fighting-type combination.
Croagunk's English name comes from a combination of the words "croak" and "gunk". Its Japanese name, グレッグル Gureggru, comes from a combination of "ケロケロ kerokero", an onomatopoeia for a frog's croaking, and "げろげろ gerogero", a Japanese word meaning disgusting.