This Croagunk is a poison/fighting-type Pokémon owned by Brock. It is the first Pokémon Brock acquired during his Sinnoh journey.
In its initial appearance, Croagunk was seen as a loner before being taken in by Jessie to fight opponents in their unofficial gym battles. After Team Rocket betrays it, Croagunk appears to show interest in Brock and began its close relationship with it. When Brock flirts with other girls even his crushes of the Nurse Joys and Officer Jennys, Croagunk uses Poison Jab on either in Brock's chest or his back leaving him stunned to dragged him away. This act is similar to Misty and Max when they drag Brock to his ear when he flirts with girls.
In O'er The Rampardos We Watched!, Croagunk also has a grudge on Team Rocket after they betray and abandoned it from its debut and it demonstrates by using Poison Jab on Jessie's stomach causing James and Meowth to carry her in a stunned state to escape in fear. Croagunk is also expert in posing other Pokemon during the dress-up contest in All Dressed Up With Somewhere To Go! where he poses as Politoed and it is main reason for Brock to win the Pokemon egg.
Brock and Croagunk first met at an unofficial Pokémon Gym run by Team Rocket. They quickly gained a close connection; indeed it has been said that Croagunk is about as close to Brock as Ash is to Pikachu. Whenever Brock fell for a girl, Croagunk would use Poison Jab on him and drag him away. Croagunk actually seemed to enjoy doing this to Brock despite his close relationship with him.
Croagunk shared a very fierce rivalry with Saturn's Toxicroak which started the moment they first saw each other. It is unknown exactly why Croagunk hated Toxicroak so much but its rivalry with its evolved form was so strong that it would often ignore Brock's advances on women. Croagunk was once seriously injured by Toxicroak, who smashed him through a skylight and then stabbed him in the stomach with one of its claws. Croagunk eventually got his revenge during his final encounter with Toxicroak, smashing a Brick Break onto his evolved form's head and defeating it, ending their rivalry. Some people say that Croagunk has a crush on Cynthia.