Puffy Fluffy is a nudibranch who 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
Orange blushes on both cheeks
One small triangular tooth
Does not get along with other pets
Long, giant, slender, and slug-like (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)
Role in series
"A Pal for Gary"
SpongeBob buys Puffy Fluffy so that Gary will not be lonely while he is at work. He takes him from a fortune teller, despite her warnings about how 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 attacks him in self-defense and to save SpongeBob. Despite saving his life, SpongeBob blames Gary for making Puffy Fluffy run away, even though the pet was dangerous and nearly ate both of them.
What's Your Bikini Bottom Pet?
He is one of the pets featured on the title screen in this online game.