|This article is about a/an kaijin in Kamen Rider Black.|
In the first battle, Ammonite Mutant used its telekinetic powers on oil drums to attack Kamen Rider BLACK. He punched off its head which turned into a small ammonite. It was found by a boy named Shigeru (who was visiting the grave of his pet Pochi) who named it Marumo, eventually learning of its psychic power and brought it whatever it needed. With the Three High Priests were going to punish Ammonite Mutant for losing his head, Bilgenia appeared and revealed that Ammonite Mutant had two brains as the fighting brain emerges, intent to find its missing head. When Ammonite Mutant appeared, Kohtaro got Shigeru to safety. Ammonite Mutant caught up with Kohtaro and Shigeru at Pochi's grave. As Shigeru ran, Kohtaro became Kamen Rider BLACK and fought Ammonite Mutant. Shigeru ran into Bilgenia as Kamen Rider BLACK split its second head open just as Bilgenia appeared with Shigeru in his arm and threw its head back at Ammonite Mutant and its psychic powers are active once more. Battle Hopper attacked Ammonite Mutant and Kamen Rider BLACK used it to attack the rock that Ammonite Mutant had control of. Kamen Rider BLACK killed Ammonite Mutant with his Rider Kick (which also removed the psychic head from it again) and its head awakened Shigeru before disappearing.
With the first brain, it has telekinetic abilities which it uses to manipulate objects, and it can attack with its tentacles. With the fighting brain, this monster can headbutt.