Swimmy was the original prize of the show, who was won by Bowling Ball, which later became his pet.
In That's A Tree, Bowling Ball wins the show, since everyone else quit it. It is later reviewed that the forest is actually a tree. Beach Ball suggest to name it. BB named it Swimmy. He's not later seen in the episode.
In Parallelogram, Swimmy appears along with The Winner, Envelope and Pear. Pear asks if the other three wants to go to a paranormal dimension, which BB agrees on. After the intro, Pear and Envelope talk to eachother, while Bowling Ball walks around with Swimmy on his head. BB then goes into another dimension, but his pet is gone. After a talk with Mustard. he goes back into the previous dimension. Swimmy begins to move slowly, but suddenly he starts to run away from his owner. After the sun goes down and BB gets tired, Marker appears and teleports them back into the home world.
In Color Bomb, Gumdrop wants to show The Original Winner her new pet, which turns out to be swimmy. They have a talk. Later, in another dimension, Gumdrop shows BB one of his ugly eyes and that she drew something on Swimmy. LATER on, he's one of the judges in the challenge.