Monstro
Monstro is a character and world from Kingdom Hearts and Kingdom Hearts: Chain of Memories.
Monstro, though technically not a world, acts like a world in the first two Kingdom Hearts games. Monstro was the villain of Pinocchio.
Monstro is a whale, and so requires water. However, he apparently can survive in space, as he devours the Gummi Ship on their way to Atlantica or Halloween Town. Monstro is so enormous that its playable map is bigger than that of some worlds, like Wonderland. "Pinocchio's World" itself was destroyed along with the Pride Lands and other worlds.
The boss inside Monstro is the Parasite Cage. The Watergleam Summon Gem is found within Monstro.
If one leaves Monstro and tries to go to the different direction, then they will be caught by Monstro again and Monstro will appear twice on the map.
