Direwolf
From A Wiki of Ice and Fire
The direwolf is a close relative of the wolf, but larger. According to Theon Greyjoy in the first chapter of A Game of Thrones, direwolves hadn't been sighted south of the Wall for two hundred years, until Robb Stark found a dead female direwolf with a litter of six direwolf pups on the path between the holdfast where Gared was executed and Winterfell. A grey direwolf in a white field is the banner of House Stark.