Ooze Monsters (also known as the Mutants), also called Oozing Creatures, are slimey, oosing creatures that first appeared in "Simon and Marcy". Their behavior slash the animalesco and his intentions with other living beings are unknown. They don't own nor sense reasoning of the communication. They are easy to beat with any blunt object; they are, however, an extremely dangerous group. Ooze Monsters are presumably attracted to loud noises, because when Simon knocks over the Clambulance, it makes a loud noise, attracting all the monsters to attack him and Marcy. They are frozen when Simon puts on the crown and creates a snow storm.
They are later seen in "James," where a large group of them are found in a pit in the Desert of Wonder (which is, apparently, the wreckage of the town Simon and Marcy lived in) where they attack Finn, Jake, Princess Bubblegum, and James. In the end, James sacrifices himself so the others can escape, and is turned into one of them, and is shown leading the others out of the pit. In "James II" they attack the Candy Kingdom, but they are defeated by Gumball Guardians and Jameses.
- They have a humanoid appearance, inside they have a viscous liquid green color, coming out from the holes in his body. They also have some resemblance to the Candy Zombies.
- When PB states that the Ooze Monsters are "from another time" the exact time would be 996 years ago.
- Since "Simon and Marcy" is the earliest we know of the world post-Mushroom War, the Ooze Monsters are likely one of the earliest recorded (if not the very first) mutations caused by the Mushroom War.
- The ooze is likely the same substance as seen in "The Vault" (when Shoko fell into the green ooze river), and also in "Mortal Folly" and "Mortal Recoil" (when Princess Bubblegum fell into the Lich's green ooze), and also in "Jake the Dog".