Because of the naturally mutagenic properties of blob-things, and due to the way evolution works, it was bound to happen that out of the 200,000 extant species of walrus, of which each contains a population of around 650,000,000,000, there would be one Awesome mutant born with superior skillz and powerz.
At his first form, he's just a damn walrus. Like the ones at the zoo. Lame.
At his second form, his size increases dramatically to world-threatening blob-thing status, and his teeth disproportionally large to its body, even for a walrus.
At its third form, it develops sentience, clairvoyance, compound eyes, lazer rays, sharper teeth, three hidden vaginas, and even more blobliness.
It was at this point the Revengerists had to come in and destroy it, even though it just takes a ridiculous amount of hit-points and could latch on to your face so you'd have to toggle, seriously really annoying villain.
The Sabretooth Walrus eats mostly penguins, fish, ducks , and humans, just like all other walri.