Croagunk is a dark blue, bipedal Pokémon that resembles a poison dart frog. It has yellow eyes, which are surrounded by black markings. There are orange, expandable pouches on its cheeks, and its teeth are wide and flat. A black marking encircles its upper arms and chest, and there are white bands around its chest. The white markings are slightly higher up on the female Croagunk than the male. It has three fingers and toes, most of which are black. The only digits that are not black are its center fingers, which are orange instead.
Croagunk is a mild and docile Pokémon despite the unnerving noise it makes, and only fights when it has to. But when it does fight, it uses its eerie croaking voice to distract its opponent while stabbing them with its fingers.
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.