Brock's Croagunk was the first Pokemon that Brock caught in Sinnoh.
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 feirce 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 it's 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 forms head and defeating it, ending their rivalry.