Monstro later reappears in Kingdom Hearts Birth by Sleep Final Mix as an optional boss in the Monster of the Sea battle in the Mirage Arena, and in Kingdom Hearts 3D: Dream Drop Distance with his homeworld, Prankster's Paradise. In the latter, his inside is playable again.
Settings and Areas
Monstro is so enormous that its playable map is bigger than that of some other worlds, such as Wonderland. If one leaves Monstro and tries to go a different direction, then they will be caught by Monstro again and Monstro will appear twice on the map. Sometimes, attempting to return to Monstro will not work, as Monstro will temporarily leave his spot; Goofy will express his relief at this and the party will have to come back later.
Monstro has one of the most confusing layouts of any world; it is essentially an interconnecting maze. It takes a lot of patience and a little ingenuity to make your way through. Many openings open into different parts of the same room, allowing for much confusion of anyone trying to work their way through. As the surfaces in each area look the same, the only visual differences in the Chambers are the ledges, the treasures, and the occasional Barrel and debris found in them. In addition, the Chambers are marked only with numbers and are not traveled through in numerical order. Finally, Monstro defies anatomy as it is known, so natural assumptions such as going "down" to reach his stomach, for example, do not hold (you have to climb up his throat).
Sora and his party enter through the Mouth (口, Kuchi?), the largest room in the world. Here, among the piles of wood and shipwrecks, the party will come across Geppetto and Pinocchio, as well as the world's Save Point. Troublesome Pinocchio leads the party down the into the whale's innards. Chamber 1 (腹部1, Fukubu Ichi?, lit. "Abdomen 1") is the arrival point for the party here. The lowest door leads to a dead-end at Chamber 3 (腹部3, Fukubu San?, lit. "Abdomen 3"), marked by a large piece of debris blocking off the other lowest exit. The door on the ledge of Chamber 1 will take the party to Chamber 2 (腹部2, Fukubu Ni?, lit. "Abdomen 2"), which leads to Chamber 3 going down, and to Chamber 5 (腹部5, Fukubu Go?, lit. "Abdomen 5") going up. Chamber 6 (腹部6, Fukubu Mu?, lit. "Abdomen 6"), which comes soon after, stands out because it is formed like a set of stairs. Chamber 5 will also send the party to Chamber 4 (腹部4, Fukubu Yon?, lit. "Abdomen 4"), from which the Bowels (腸, Chō?) and the first battle with the Parasite Cage can be found, as well as another Save Point.
Returning to the Mouth and using High Jump, given by Geppetto, the party can at last reach the Throat (のど, Nodo?), a tall, pink chamber with a series of ledges to ascend. At the top, the party can reach the Stomach (胃袋, Ibukuro?) and the second boss battle against the Parasite Cage.
In Kingdom Hearts: Chain of Memories, there are extensions coming out of the floor that can be hit or jumped on to gain items. There are also extensions from the walls that can cause damage to Sora or Riku if approached; these can be hit to gain HP orbs.
In Kingdom Hearts 3D: Dream Drop Distance, much of Monstro's settings are based directly off the digestive system. The opening area is Mouth, similarly designed to its Kingdom Hearts counterpart however the boat where Geppetto resides on is on the left side of the mouth. Beyond here is the Gullet, a large room with a number of ledges and numerous piles of wood. Following this is Belly, a room with numerous smaller chambers, the largest part of this area has a HP-depleting acidic floor, much like a human stomach would have with hydrochloric acid. After the belly is the Cavity, a large blue-colored room that can be flipped upside down with the numerous uvulas dotting the area, there is also a long-winding path that leads to the Bowels, a rectangular area that is mostly barren with the exception of two uvulas that allow you to flip the room.
Sora, Donald, and Goofy are flying away from Agrabah to get to their next destination, when they spot something huge flying towards them on the Gummi Route. They quickly realize it is a massive whale, but cannot get out of the way before the beast swallows their ship whole.
Inside Monstro's flooded mouth, the trio finds Pinocchio again and meets his father Geppetto in a broken ship. While speaking with Geppetto, Pinocchio slips off to explore Monstro. Sora and his party quickly run after the puppet into the bowels of Monstro. There, he finds Riku; the two friends exchange some quick words before Riku runs off with Pinocchio, hinting at Riku's newfound darker nature.
Sora, Donald, and Goofy chase the two through the whale, until they eventually finds them in a large circular chamber: the bowels of the beast. Pinocchio is taken captive inside the Parasite Cage Heartless. Sora and Riku team up to take down the Heartless, but are unable to completely destroy it. Riku takes Pinocchio; his words hint to Sora that there is something different about his old friend. Sora tracks the puppet to the Stomach, where Riku and the Heartless attempt to take Pinocchio's heart, but fails. Riku flees as Sora enters the belly of the beast where he defeats the Parasite Cage once and for all and rescues Pinocchio.
From all the commotion in his organs, Monstro spews out the Gummi ship and the trio to prevent any more trouble. Pinocchio and Geppetto also escape and set up a shop in Traverse Town.
Kingdom Hearts: Chain of Memories
Sora, Donald, and Goofy visit a recreation of Monstro in Castle Oblivion. There, the events of Sora's first visit to Monstro are reenacted, minus Riku's involvement in the events that transpired. Both the chase for Pinocchio and the puppet's reunion with Jiminy are very similar. The party again fights the Parasite Cage, though only once this time. Pinocchio is told to be brave, his struggles eventually getting him out of the Heartless's maw-like belly. Sora, Donald, and Goofy also fight with several Shadow Heartless ensues in order to make Monstro spit out Pinocchio and Geppetto, as per the puppet's suggestion that a big enough struggle would cause the whale to sneeze. They succeed.
Meanwhile in the Basement, Riku explores the replica of Monstro, encountering and defeating the Parasite Cage.
Kingdom Hearts 3D: Dream Drop Distance
After learning that Geppetto has been swallowed by Monstro and that Jiminy and Pinocchio has jumped of a cliff into the ocean to follow, Sora quickly goes off to rescue them. Once he's gone underwater, he soon finds Jiminy and Pinocchio, swimming away from Monstro. However, the force of the water pulls him into the chaos, and Pinocchio and Jiminy are then swallowed by Monstro. Sora gives chase, but a Dream Eater appears and freezes Monstro in an iceberg, which floats to the top of the ocean. Sora follows and fights the Chill Clawbster Dream Eater on the clusters of the frozen Monstro. During the battle, Sora frees Monstro, encasing him in a bubble. The Chill Clawbster is then defeated and attempts to flee, but is then swallowed by Monstro. Sora falls into the ocean and is blown away when Monstro sneezes at him. Sora wakes up on a beach, assuming that Prankster's Paradise was destroyed not long after Monstro swallowed everyone, and also recalls how they soon escaped and ended up in Traverse Town. The Keyhole then appears and Sora unlocks it.
Riku finds himself inside Monstro and spots Geppetto, who is looking for Pinocchio. After hearing the situation from Geppetto, Riku offers his help and begins searching for Pinocchio with Jiminy, who informs Riku that Pinocchio wandered of with a man in a black coat. They eventually catches up with Pinocchio and the unknown in the black coat. Pinocchio apologizes to Jiminy for just vanishing, and Jiminy notes that by being aware of his own mistakes his starting to learn what it takes to be a real boy. The unknown removes his hood to reveal himself to be Riku and disappears into corridor of darkness. Jiminy questions what he just witnessed as Riku explains that he gave into the darkness once and it has come back to haunt him several times ever since. Pinocchio and Jiminy proceeds to ask if he's got any friends he can rely on when times get tough. He smiles and says that he has got one friend in particular who he could never do without. They then hear a massive roar shaking the inner areas of Monstro. Riku goes to investigate and orders Jiminy and Pinocchio to head back to Geppetto.
He finds the Char Clawbster Dream Eater and destroys it. Afterwards, Riku watches as Pinocchio and Geppetto is happily reunited. He ponders on the fact to why this world was trapped in a past version of itself and remembers Yen Sid's words of wisdom. Once the Keyhole reveals itself, Riku unlocks it.
Note: After rescuing Kairi from Hollow Bastion, Monstro will begin collecting sets of Heartless from every world except Atlantica, Hollow Bastion, and End of the World. Each time Monstro is entered at this point, it will have its normal set of Heartless, as well as the Heartless set of a different world. The auxiliary set will change each time Monstro is reentered.