Monstro the Whale is a minor villain from the Kingdom Hearts Unlimited Saga, primarily reprising his role from the first game and Dream Drop Distance, only now his world of Prankster's Paradise is still accessible for adventuring for the first "game"/quest.
In the first adventure, Monstro is a giant whale controlled by the Coachman Barker to serve as a means of capturing runaway children from Pleasure Island and return them back for future usage in the black market, but after the Coachman is slain in battle, Monstro goes on a rampage and begins swallowing nearly everything in his path, including Pinocchio, Geppetto, and the Keyblade Gang. It is inside the whale's stomach that Mickey and Oswald first begin to drift apart from one another and start becoming enemies.