Fungus Humungous was a giant mutated mushroom, who was capable of causing people to experience their worst fears. Leo encountered the mushroom in the episode of the same name, and seemingly destroyed it as well, using sunlight. Later Mikey brought mushroom pizza into the lair as a joke, but some spores from the mutant got into to the mushrooms, bringing them to life, as Mikey finds out when he opens the box. It is unknown what happened to the Fungus Humungous after that.
It was created when a canister of mutagen from The Mutation Situation fell into the sewer and mutated a colony of fungus. The mushrooms' first victim was a frightened construction worker who recently encountered the Squirrelanoids. Using their spores, they made him think everything that moved was a Squirrelanoid.
It mainly consists of a large mushroom cap, with a large center eye, and several other eyes that can be deployed. It's base consists of a swollen mass of fungus with a canister of mutagen embedded in its "chest". It also has several glowing tendrils protruding from the ground around the base.
It's minions have disk-like heads with 3 equidistant yellow eyes on the cap. They also have small, slender bodies with thin, tendril-like arms and legs. The cap can expand to form a ball around it, allowing quicker movement. The top "bump" of the cap opens to spray the spores.
- Fungus Humungous is conceptually similar to IDW Publishing's version of Shub Niggurath, originally a Lovecraftian entity reimagined that appears in IDW TMNT's Infestation 2 miniseries. This version of Shub Niggurath is not actually a god, but a giant slowly-dying fungus that has been festering beneath New York City for at least decades and possibly centuries, requiring a scarce supply of animal and human sacrifices to keep him nourished. Unlike Fungus Humungous, Shub Niggurath isn't specifically identified as a mutant himself, but the people and animals he infects to serve his will are deemed "mutations."