|This article is about a/an kaijin in Kamen Rider Black.|
Rat Mutant was first seen when the Three High Priests became Grand Mutants.
At the time when Ryuzaburo Sakata established the EP Party for world peace, Gorgom was using a stolen invention that would heal humans but would kill them if overused. To keep Kohtaro from getting to it, they unleashed Rat Mutant and had it pose as someone in a group against the EP Party as their inside person. Kohtaro exposes the double agent who sheds his disguise and those against the EP Party went up against Grand Mutant Baraom and the EP Followers. Kohtaro transforms into Kamen Rider Blakc and fights Rat Mutant. After using his Multi-Eye to read Rat Mutant's movements, Kamen Rider Black chops off Rat Mutant's tail. Once Kamen Rider Black killed Rat Mutant with the Rider Kick, the soldiers for the EP Party regressed back to rats and Kamen Rider Black saves the rebels from Baraom.