Puffy Fluffy is a character who first appears in the episode "A Pal for Gary" as the main antagonist. He also appears in the online game Gary's Revenge.
Puffy Fluffy is a nudibranch and can transform into a vicious snake head fish/Dunkleosteus form. In each version, he is very different.
Big shiny light purple eyes
Cyan fins and tail
Pink and puffy lips
Light green fluffy body
Pink blushes on both cheeks
One small triangular tooth
Does not get along with other pets
Long, giant, slender, and sluggy (round) light green body
Long, jagged teeth
Multiple tongues with mouths inside
Light blue giant fins and tail
Tongues tend to be sensitive to pain (as seen when Gary yanks the tongues)
SpongeBob buys him so that Gary will not be lonely while he is at work. He takes him from a Nudibranch seller, despite her warnings that he is unstable and dangerous around other pets. SpongeBob names him Puffy Fluffy and is angered with Gary's scared behavior towards his new "friend". SpongeBob fails to notice anything wrong with Puffy Fluffy, despite Gary acting very abnormal around him. SpongeBob blames Gary for everything the new pet does, even when seeing him in monster form when Gary is wrapped up by his tongues and almost gets eaten. Despite almost being eaten too, SpongeBob remains oblivious towards the situation.
Puffy Fluffy ends up running away after Gary ties up his tongues together with a leash. As a result, SpongeBob blames Gary for making Fluffy run away, despite the pet being dangerous and nearly eating them.
Nudibranchs, which is what Puffy Fluffy is, are actually sea slugs.
His multiple tongues might be a reference to the Graboids from the sci-fi film Tremors.
When getting Puffy Fluffy from the gypsy, she says something about the pet being dangerous. This is possibly a reference to the movie Gremlins, in which a man buys a strange pet from a Chinese shop owner, who refuses to sell it to the man because it is dangerous.
Nudibranchs appear as the main antagonists in the online game Gary's Revenge.
Puffy Fluffy's monstrous and vicious behavior is actually warned to SpongeBob by the gypsy selling the nudibranchs, but he completely ignores her and takes one without even paying.
Puffy Fluffy tries to eat SpongeBob at one point, which is appropriate since real nudibranchs eat sponges.
Due to the vivid resemblance of Puffy Fluffy to mogwai (and gremlins), it is probably midnight or after midnight when he transformed (because of eating). This is the same case as the mogwai from Gremlins.
In Croatian dub, his name is "Čupavko" which means "Hairy."