During the 2012 Winter event, Jagex Moderators had the ability to spawn multiple Fenris wolves which could then be attacked. While they could be spawned anywhere, they were most commonly found at the Grand Exchange.
Before the quest Fur 'n' Seek was released, they were simply called "Wolf".
In Norse mythology, Fenrir is known as the "Fenris Wolf", and is one of Loki's 3 monstrous children along with Jormungandr (the Midgard Serpent) and Hel. The Gods will attempt to bind Fenrir, the first two shall fail, the third Fenrir will be shown the magical ribbon Gleipnir, Fenrir agrees as long as one of the gods is brave enough to place one of their hands in his mouth as they bind him. Tyr is the only one who accepts the task. As Fenrir struggles Gliepnir will tighten and Fenrir will bite Tyr's hand off in rage.
In Norse mythology, Fenrir shall break free from Gliepnir and travel to Vigrid to fight in Ragnarok. There he shall swallow Odin whole. He is also the father of Hati and Sköll.