Fenrir
One of the three children of the trickster god Loki and his giantess Angrboda, Fenrir took the form of a wolf. He and his younger brother Jormungand were feared by the Aesir gods*.
The three were brought to Asgard, the home of the gods, from their mother’s realm Jotunheimar. The other two, Jormungand and Hel, were thrown out into the ocean and Niflheim, the realm of the dead where they rule until Ragnarok.
Odin* grew very afraid of the prophecy that foretold these children would bring harm to him. But he also felt that Fenrir was a good animal, and so he tried to keep him safe.
Fenrir was very dangerous, though, and the Aesir gods did not think that he would break free from his fetter easily. The prophecy was that if the wolf were to break loose he would kill Odin and avenge him at Ragnarok.
But the wolf was not strong enough to break his bonds, and so they chained him to a rock. They used a chain that was made of soft and silken string. The gods tried to break it, but they could not.
Eventually they tried a stronger chain, one that was not so soft and silken. This time, only Tyr had the strength to bind the wolf and keep it from breaking his bond. But, he also lost his right hand in the bargain. The wolf did not break his fetter, and so the AEsir gods continued to watch him.