Vicious Dogs appear to be a form of Pitbull in Fallout 3. They are hairless, starved dogs that will attack anything in their sight. They are weak creatures on their own, but when they travel in packs of 4 or 5 they can be dangerous to someone who's injured or at a low level.
Some Vicious Dogs have been taken in by Raiders or Talon Company Mercs and used as guard dogs. If you have the Animal Friend Perk, Vicious Dogs will no longer be hostile towards you. If you have the second rank of the Animal Friend perk, any nearby Vicious Dogs will come to your aid in battle.