|This article is about a/an kaijin in Kamen Rider Black.|
Mushroom Mutant assisted High Priestess Bishium into messing with a girl named Yuki by entering her dreams. This plot later thickened when Bishium disguised as a nun fed a mushroom soup (which would kill them if the body rejects it) to Yuki and other children to turn them into Cyborgs of Hate without surgery and using the mushroom pollen to erase their memories that would turn them into demons without memory of their parents. Kohtaro found Yuki wandering on the road at night only for him to get attacked by Mushroom Mutant. Kohtaro transforms into Kamen Rider Black and fought Mushroom Mutant. Kamen Rider Black was hit by one of Mushroom Mutant's mushrooms before he escapes which resulting in a nightmare Kohtaro had. Later that night, Yuki and Kohtaro were transported to the Realm of Gorgom where Mushroom Mutant attacks Kohtaro. Kohtaro transforms into Kamen Rider Black and fights Mushroom Mutant. During battle, Kamen Rider Black ends up in a pollinated mushroom area and is attacked by Mushroom Mutant and Bishium. Kamen Rider Black tricked Bishium into attacking Mushroom Mutant. Once Mushroom Mutant was killed by the Rider Kick, Bishium's illusion was negated and the effects on the pollens wore off.
It can burrow underground, and detach the pollen-covered mushrooms which target one's nerves.