Japanese people aren't famous for their faithfull adaptation of Northern mythology. Of mythology in general as a matter of fact

Here's some backstory about the original mythical Fenrir: He's the son of Loki, brother of Hel (the goddess) and Jormungandr (Yeah, THAT one). He was fated to grow super strong and basically fuck everything up. The Norse gods panicked and kinda tried to trick him into getting bound in chains. They tricked him into thinking that their binding him was a test of his strength. The first two sets, both forged by the gods, were broken swiftly, because Fenrir was fucking strong and ridiculously huge. He was big enough that if his mouth was open, the top would reach the sky, and the bottom would touch the ground. At the same time. Then they had the dwarves commission Gleipnir, a chain made of six impossible things: the root of a mountain, the spittle of a bird, the breath of a fish, the sound of a cat's footfall, the sinews of a bear, and the beard of a woman (I always laugh at that one).

Fenrir noticed that Gleipnir looked different than the rest of the bindings, and said that if he was going to be bound in them for this 'test of strength', someone would have to put their hand in his mouth as insurance. Tyr, the war god, had the brass bearings to do it. When Fenrir couldn't break Gleipnir, he bit Tyr's hand off. Thus, he would stay bound by Gleipnir until Ragnarok occurred. Then he'd kill Odin.

Of course, now we get this kinky little puppy.