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."