Buffalo-Kaijin, also known as Buffalo Mutant, is a monster created by the secret organization Gorgom. As his name suggests, his design and powers are based on a buffalo, granting him a tremendous amount of brute force and the ability to ram through obstacles with ease. Beyond his physical strength and human-like intelligence, his primary ability is possession; he can invade and control the bodies of human beings to further Gorgom’s objectives.

In his role within the story, Buffalo-Kaijin is dispatched by Gorgom to execute a plan designed to erode trust among humans. He targets a young girl named Mami, possessing her to dramatically enhance her athletic abilities in sports such as baseball and soccer. While controlling her, he uses her body to violently beat up a child who mocked her, deliberately causing concern for her parents. He then manifests himself as a mysterious man to whisper manipulative suggestions to Mami, specifically that adults cannot be trusted, deepening the psychological and social disruption.

The protagonist, Kohtaro Minami, is forced to confront Buffalo-Kaijin while the monster is still possessing Mami. This creates a significant disadvantage for Kohtaro when he transforms into Kamen Rider BLACK, as he cannot attack with full force without harming the innocent girl. The situation reaches a turning point during a second battle near a factory. Mami inadvertently frees herself from the possession by stepping on a fallen live electrical wire, which forces the Buffalo-Kaijin out of her body. With the hostage situation resolved, Kamen Rider BLACK is free to fight, ultimately chopping off the monster’s horns and destroying him with his signature Rider Kick. There is no further information regarding any significant development, relationships beyond this tactical alliance with Gorgom, or later appearances in the series.