You raise a really good point. It is strange that there are no bears in Norse myth. The wolf seems to have really engaged them, perhaps because of the duality of wolf/dog, tame and wild, while the bear to them was totally wild.
I wonder if @LocalFluff isn't on to something, because berserks were seen as totally outside society, too unpredicatable and dangerous to have around. (In Egils saga Skallgrim kills a servant when the berserker rage descends on him, and nealynearly kills his son.) A wolf could become a dog, but you can't tame a bear and put it to use or have it as a pet.
By the way there were a kind of berserkers, ulfhednar, who turned into wolves, although you don't hear so much about them.
PS - The Lewis chessmen include berserks, biting their shields.