Kingdom Hearts
The Parasite Cage was a Heartless sent into Monstro to find and capture the heart of Pinocchio (and possibly locate Monstro's). It captured the puppet when he ran away from Riku. However, before it could succeed in escaping, Riku, Sora, Donald, and Goofy battled it in Monstro's Bowels. Eventually the Parasite Cage spit out Pinocchio and climbed away into Monstro's depths. Despite being defeated, the Parasite Cage retained Pinocchio's heart.
Sora chased Riku into the Stomach, only to discover that the Parasite Cage had lodged itself there. With Pinocchio's heart, the Parasite Cage had increased in size, and had become much more powerful. Sora battled it one last time before it was finally destroyed.
The Parasite Cage's main body is a large, magenta blob with glowing yellow eyes and thin, rounded teeth that resemble prison bars. This body rests on a flower-like base that is violet and aquamarine. A much smaller pink head rests on top of the main one. This second head sports a jagged mouth, glowing yellow eyes, two short, dark purple spikes under its mouth, and a curled, magenta antenna on its head. Two long, aquamarine tentacles sprout from either side of the second head and end in violet "hands" with three fingers, each with a dark purple spike on it. Its Heartless emblem is between its eyes.
In Kingdom Hearts Final Mix, The Parasite Cage's main body is light orange, as is the antenna on the smaller head. The smaller head is light tan and the small spikes on it and its hands are blue. Its arms and base are pale green, its hands are yellow, and the flower-like section of the base is violet and lavender.
The Parasite Cage's name reflects its apparent great appetite and the fact that it is fought inside another living creature. "Cage" clearly references its mouth, which resemble prison cell bars, and its ability to capture opponents. Its appearance is also similar to the Prison Cage from Final Fantasy IX.
 Kingdom Hearts
The Parasite Cage mainly uses its tentacles to attack opponents. It attacks by swinging them or slamming them on the ground. It can trap enemies in the mouth on its body. It can also suck Monstro's stomach acid through its tentacles and spit it out through the top head.
 First battle
At the start, remain on your platform and lock on to the Cage’s head, shooting Fira or Gravity spells at it. Keep doing this until it’s beaten and escapes. If you need to recharge, either use an Ether or attack the head and cage with the Keyblade until you’ve regained enough of your MP. Riku will aid you in this battle, making him the only fellow fighter in the game. After winning, Goofy learns the Cheer ability. Note that due to Riku aiding you in this battle, it will be easier than the second fight. Also worth noting is that if you complete the first run through Hollow Bastion, Riku will not actually appear. This means that he will not be there to help you.
 Second battle
This fight is much the same as the last one, but the Cage has developed a new poison breath attack. When the Cage gets tired and sags backward for a moment, its mouth will open. Bash it with the Keyblade, and Try to execute a couple of flying combos during this time. When it recovers, return to your normal strategy of casting spells at the head. Eventually, it reverts to its old strategy, and the player can get closer to smash it with the Keyblade. After the battle is over, Sora and Donald will learn the Stop spell. The party is then automatically ejected from Monstro and returns to the Gummi Ship.
 Kingdom Hearts Re:Chain of Memories
In both Sora's and Riku's battles with the Parasite Cage, there will be four revolving platforms, floating on digestive acids. Using the Gimmick Card will cause these four platforms to disappear, and a large one to appear in the center, close to the Parasite Cage.
Since you obtain the Monstro world card within the first few stages of the game, defeating the Parasite Cage is a cinch. The only thing you need to do is successfully fill the Dark Points gauge bar up to 30 by breaking every single card. After that, attack the Parasite Cage with all you've got—simple attacks, sleights, or anything else. Also note that while Riku is in Dark Mode, HP orbs come out of the Parasite Cage with every attack, so it is easy to heal, and easy to stun the enemy. In addition, you have the Search Ghost enemy card to heal you each time you strike it.
 Notes and references