In his 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 him, Croagunk appears to show interest in Brock and began his close relationship with Brock. 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, his back or most likely his butt 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 him from his debut and he 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 Pokémon during the dress-up contest in All Dressed Up With Somewhere To Go! where he poses as Politoed and it is the main reason for Brock to win the Pokémon egg. Therefore, in Enter, Galactic!, Croagunk has a very strong rivalry to Saturn's Toxicroak which is Croagunk's evolved form upon meeting and has a fierce battle with each other. This rivalry is also carried by Ash's Buizel when he has a strong rivalry with Crasher Wake's Floatzel which is Buizel's evolved form.
In Cream of the Croagunk Crop!, Croagunk is very competitive and focus during the Croagunk Festival as Crasher Wake was impressed how outstanding Croagunk is especially during the championship challenge with another Croagunk, Craig to show how powerful Croagunk is.
It seems that Croagunk has a weird personality as he sometimes will look at his own reflection in mirrors or just standing alone. He hates sweets as well, since he always shows a disgusted appearance whenever he eats one.
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.
In the episode Cream of the Croagunk Crop!, Croagunk becomes very competitive in the Croagunk Festival held in the Great Marsh. Crasher Wake and Nurse Joy was very impressed how outstanding it is even the Brick contest where it manages to break all the bricks with a single Brick Break. Therefore, in fighting against the champion which is a Croagunk nicknamed Craig. Croagunk manages to beat it with Poison Jab multiple times only to be interrupted by a giant robotic Croagunk manned by Team Rocket. Croagunk does its best to save all the captured Croagunk but it was rescued by Nurse Joy's female Croagunk named Chrissy.
After Team Rocket is blasted off by Ash's Pikachu, Crasher Wake declared Nurse Joy's Croagunk the winner as Croagunk didn't mind winning the festival after it saves all the Croagunk from Team Rocket.
Sun & Moon Series
During Ash's trip during Kanto, Brock left Croagunk at Pewter Gym.
At Pewter Gym/Home
Caught for someone else