 This meant they were supernatural and special, both qualities that Mouse has shown as he has attained his massive mature size.While Harry returned all the Foo puppies he could find, one particularly scrappy pup hid under his car seat and he was unable to return it.Mouse grows startlingly large by the start of "Dead Beat", with his shoulders reaching nearly to Harry's waist, and kills the former Denarian Quintus Cassius in order to prevent him from killing Harry.