Ruk, like his brethern, was a regular walrus before being mutated by the remaining half of Ragnarok's magic stone. Ragnarok punished the mutant walruses by killing them, leaving Ruk as the sole survivor, to his knowledge.
As Ragnarok's melting of the Arctic began flooding the Earth, the Tortugas Ninja and Moycat headed up to check it out. In a cave, they found Ruk being whipped by a slavedriver. Moycat got angry and knocked the slavedriver out in one blow, saving Ruk. Ruk told them his story and they went out to find Ragnarok. Ragnarok blasted the group using his stone half, but Ruk found the other half. Ragnarok blasted him, too, though, and damaged badly, knocking the stone half out of his hands.
After a bit of clashing and keep-away, Ragnarok reunited the two halves of the stone and grew to massive proportions. He was thrwarted by the sudden appearance of the Norse god Balder, who froze Ragnarok solid. Donatelo asked for Balder to heal Ruk, which he obliged. Ruk requested to stay a mutant to protect his species, and Balder agreed.