Direwolves

Direwolves are a race of large wolves that like werewolves, live outside of the Wall, though many people believe direwolves to be a myth.

Background
Direwolves have existed in secret for hundreds of years in forests outside of the Wall. According to Alec, who is an expert on the subject of direwolves, they prefer to eat bears and pigs, but will attack humans if there isn't any game around for them to hunt.

Direwolves are also shown to have a bond of some kind with werewolves, treating them as their alphas and even bringing them the remains of anything or anyone they kill.

Abilities
Enhanced Strength: Although not as powerful as a werewolf in its beast form, direwolves are strong enough to rip limbs off of humans and animals alike, as well as shatter bones with their teeth.

Vulnerabilities

 * Mortality: As direwolves are basically just large wolves, they can be killed by anything that can be kill a normal wolf or any other living being, such as when Lady was killed after Eddard Stark stabbed her with a knife.