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.
Under Brock's ownership, Croagunk takes the place of Misty and Max, pulling Brock away from pretty girls (similar style of Barbie dolls), though more harshly then before. Instead of pulling Brock's ear, Croagunk will use Poison Jab on Brock and then pull him away, seemingly enjoying it, as he laughs during the process.
Many Croagunk appeared in the episode Cream of the Croagunk Crop!, where there was a competition and festival for Pastoria City's mascot, Croagunk.
Croagunk has helped Brock to win a Happiny's egg in a dress-up contest, by acting as a Politoed throughout the whole episode.
Croagunk is found to be very "scary-looking" when facing the Team Galactic's Pokémon, Toxicroak, staring at each other, making scary face.
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.
Trivia
Croagunk and its evolved form are the only known Pokémon to have a Poison/Fighting-type combination.
On February 13th, 2007, Croagunk, along with other Pokémon images, were released revealing their names. Though because of the low quality image Croagunk's name was believed to be "Croalurk".
