Oozers

Goo Monsters are zombie-like creatures that appear in "Simon & Marcy." One appears in the soupery, and two others appear in the Clambulance. Goo 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. It is unknown what happened to them after this.

Appearance
They are all various shades of green, brown, and yellow, and blue, and have various numbers of holes in their head for their eyes and mouths, out of which oozes a glowing green substance. The ooze that comes out of them is very corrosive as it's seen melting through the glass window of Princess Bubblegum's tripod lander in the episode "James." 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."

Trivia

 * Goo Monsters are very similar to classic movie zombies as they are relatively slow moving, will relentlessly attempt to bite living things in order to turn them into monsters, can only make groaning noises, and groan loudly to alert other monsters to their prey's presence.
 * Goo Monsters seem to mainly rely on their sense of hearing. They are attracted to sounds and loud music (the "Clambulance" crashing and the "Happy Broth-day" card). They also appear to use sound to navigate (bumping into the shelves before Simon drops the Broth-day card, then reacting to the sound). However, they are not completely blind since Princess Bubblegum used the Tripod's flashing lights to help distract them in "James."
 * Since "Simon & Marcy" is the earliest we know of the world post-Mushroom War, the Goo Monsters are likely one of the earliest recorded (if not the very first) mutations caused by the Mushroom War.
 * Goo Monsters also resembles the "Hibakusha" or the Atomic Bomb Survivors since they are exposed by the intense radiation of the bomb as what happened to the victims of the Bombings of Hiroshima and Nagasaki. Goo Monsters however, are very far from the real Atom Bomb survivors since the Goo Monsters are mutated by the blast whilst the people killed during the bombings of Hiroshima and Nagasaki sustained severe burns, fractures and mutations are only little.
 * The goo monsters may be mutated army men, as their armour could have formed the green skin, and the holes could hae been made through mines and bullets.

Gallery
Mutantes