Bobo the Chimpanzee
Bobo the Chimpanzee debuted in the Second episode (airtime) of Season 1, and returned in the 13th episode. Since then he has appeared, or has been directly/indirectly referenced, in a number of episodes. He is unique in the franchise in that, while he is a recurring secondary character, he has only ever been seen during flashbacks (of Ron describing his unpleasant experiences at Camp Wannaweep).
Fun Fact: Bobo in Spanish means fool! :)