The Giant Bear played a minor role in "His Hero." In a flashback, he fought with Billy, the role model of Finn and Jake. Jake seemed especially excited about the time Billy fought it. The Giant Bear is mentioned in the lyrics of Billy's Song, apparently as an afterthought.
He looks like a round, chubby teddy bear with brown fur, and is able to levitate and float around like a balloon. He never moved any part of his body during the flashback because he was only floating. He is also extremely huge, as he dwarfed Billy and the surrounding area. He has a dark brown spot on his belly, a round nose and four extremely long, sharp upper teeth.