tanaquil:

A DIREWOLF is a close relative of the wolf, but larger and stronger. They are named after the real world direwolves, Pleistocene megafauna, which were larger than modern wolves (but not as large as portrayed in the books). Direwolves are extinct north of the wall, as they are a very large and dangerous predator, and people have probably hunted them out. A grey direwolf in a white field is the sigil of House Stark.

(requested by direwolfrising & daenelicious)