The Slime Mutant was the first villain of Scooby Doo: Mystery Incorporated. It was a humanoid creature that appeared to be bone and muscle, covered in a thick green substance. It powers included shooting a green slime from its hands that could stick to any surface and could dehydrate any living things, such as people.
It is first seen when three construction workers are at work in a sewer, and accidentally knock in a wall revealing an old cave system. Exploring, they come across a pile of green oil drums, with the biohazard on them and a sign reading "DANGER". They foolishly open a can that they hear pounding in, releasing the Slime Mutant who proceeds to engulf them with slime and stick them to the roof of the cave.
Later, Scooby and the gang are driving to school in the Mystery Machine, when the Slime Mutant blasts its way out of the sewers in front of the car. The Slime Mutant splatters the car with slime before slinking off.
After Scooby and the others explore the caves and find the dehydrated workers, the Mutant attacks Professor Raffalo, who the gang left the worker with to study the slime.
Afterwards, Scooby eats a sample of the Mutant's slime, and tells the gang that it is the strange dessert sold at Fruitmeyer's. Shaggy and Scooby get jobs as waitresses at Fruitmeyers, and let the others in after the shop owner leaves. They investigate. Daphne finds a locked closet, and leaves to find a key. Shaggy and Scooby inspect the same closet, and open it after Scooby picks the lock, revealing the Slime Mutant inside. He pursues them. Daphne comes to the closet, and accidentally locks herself inside. Fred and Velma save her, but then Scooby ans Shaggy barrel into her, knocking her through a secret way into the underground caverns. As they explore, they find another passage into the city bank. They decide that the Slime Mutant is Franklin Fruitmeyer using the guise to keep people away from his passage in order to safely rob the bank. They set up a trap, but when it is set off, a cage drops on everyone except Daphne and the Slime Mutant. The creature blasts the caged gang with his slime and then chases Daphne back into the store while the others eat their way out of the trap. The Mutant corners Daphne, and as it is about to strike, the gang, having freed themselves, hose down the Mutant with Fruitmeyer's dessert, freezing him to the wall. The police unexpectedly arrive, having be called by Fruitmeyer. They unmask him, revealing him to be Professor Raffalo. He tells the group his motive was to get money because his job as a teacher didn't pay well.
He would have been successful if not for those meddling kids.
The Slime Mutant is voiced by Dee Bradley Baker.
The next villains are the Gator People.