Puffy Fluffy

Puffy Fluffy is a character in the animated TV series SpongeBob Squarepants that only appeared in the episode "A Pal for Gary" as the main antagonist. He is a nudibranch (although not resembling an actual nudibranch at all). He first appears as a small chubby fish with a goofy expression and razor-sharp teeth, but eventually evolves into a huge lamprey-like beast with several sucker-like tongues inside his mouth.

SpongeBob bought him so that Gary wouldn't be lonely while he was at work, but he turned out to be a monster and attacked Gary several times (while SpongeBob was oblivious to the entire situtation). SpongeBob never notices and blamed Gary for the desruction of his house, which was really caused by Puffy Fluffy. Puffy Fluffy sheds his skin during the night and turns into his fully-grown monstrous form, resulting in Gary battling him throughout the house the entire night. When SpongeBob starts to scold Gary, ignoring the fact that Puffy Fluffy is now trying to devour the snail with his tongues, the monster decides to eat him instead. Gary saves his owner's life by tangling up Puffy Fluffy's extra mouths with his own leash, preventing him from eating and sending the monster running out of the house in a sobbing panic. Even so, SpongeBob still blamed Gary for chasing Puffy Fluffy away, even though Gary ended up saving his life and the house.

Trivia

 * Nudibranches, Puffy Fluffy's species, are actually sea slugs, which his monster formation resembles a bit.