Nanook is a Alaskan Malamute that belongs to Sam Emerson. He is protective of Sam, which is shown when he bites Michael on the hand in order to stop him from attacking Sam. It is implied that Nanook was able to sense that Michael was a half-vampire (mostly because he had a sixth sense about things like that).
Nanook kills Paul and consequently saves the Frog Brothers when he pushes the vampire into a bath full of holy water. Edgar even remarked to Sam that Nanook "helped a little", which showed that both he and Alan were quite proud of Nanook just as much as Sam was.