Whale Mutant is a whale monster created by Gorgom and an antagonist turned hero in Kamen Rider Black.
Whale Mutant overhears Baraom and Darom talking about how Gorgom's rise to power might pollute the oceans. When Whale Mutant was looking out at the ocean, Baraom recruited him in his plot to defeat Kamen Rider Black. Kotaro encountered Whale Mutant chasing children and managed to hold it off while the children got to safety. Kotaro then transformed into Kamen Rider Black and fights Whale Mutant.
As they are fighting, Bat Mutant reports to Darom that Whale Mutant was going to revolt. Kamen Rider Black continues to fight Whale Mutant when Baraom attacks as Whale Mutant shoots its sticky foam at Kamen Rider Black immbolizing him for Baraom to prepare to take his Kingstone.
Upon already knowing that Whale Mutant was going to turn against Gorgom, Baraom stabbed him with his tusk weapons and proceeded to impale this monster to kill Kamen Rider Black. Whale Mutant turned the tables on Baraom and hit him with sticky foam. Whale Mutant became an ally of Kamen Rider Black after telling Kamen Rider Black that Gorgom is going to pollute the oceans. Kotaro then guided Whale Mutant towards the ocean where he heads into to recover.
Upon learning of Kamen Rider Black's death at the hands of Shadow Moon, Whale Mutant searched the ocean and found his body. Whale Mutant then began to perform a revival ceremony (which was established by Whale Mutant's tribe) on Kamen Rider Black.
When a bouquet of flowers thrown in the ocean by Kyoko and Katsuki reaches Whale Mutant's cave, Whale Mutant places them on Kamen Rider Black and he returns to life. Darom then went after Whale Mutant only for Whale Mutant to be attacked by Bat Mutant. Kotaro arrives and saves Whale Mutant. When Kamen Rider Blackwent to save a child from Bat Mutant, Whale Mutant ended surrendering to the worshippers of Gorgom, hoping it would be atonement for being part of Gorgom. Darom used Whale Mutant as bait for Kamen Rider Black.
When Darom has Kamen Rider Black bounded by his attack, Whale Mutant uses its sonic attack to throw off Bat Mutant's radar. Whale Mutant hits Bat Mutant with its sticky foam and Kamen Rider BLACK protects him from Darom's attack. Still wounded from Bat Mutant's attack, Whale Mutant guides Kamen Rider BLACK to the cave that is Gorgom's headquarters.
Whale Mutant was attacked by Stickleback Mutant under Shadow Moon's orders. Kotaro catches up as Stickleback Mutant starts to overwhelm Whale Mutant. Kotaro transforms to Kamen Rider BLACK to help Whale Mutant only to be pulled toward the Creation King's palace. Before he was killed by Stickleback Mutant, Whale Mutant told Kamen Rider BLACK to protect the oceans. Kamen Rider BLACK would later avenge Whale Mutant by destroying Sticleback Mutant with his Rider Kick.