Dogs and wolves have been straight ahead biters for pretty much all of D&Ds and it's descendants history but I started thinking about the way a dog finishes off it's prey (at least the smaller prey) with a vicious shake.  So to give wolves, hell hounds, war dogs etc. a special bit of damage in line with the hug of a bear, rending of the carnivorous ape, or the back claw rake of a big cat how about if on a 18 or better the dog shakes for an additional bit of damage equal to or even double it's normal bite damage?  This would only apply to creatures smaller than the dog or wolf.  Since Hyperborea doesn't host a great deal of gnomes or halflings most PCs would be pretty safe from this attack from normal dogs and wolves but it would give dire wolves, hell hounds and winter wolves one more reason to be feared.