I *love* your story. It's so much more than a wank-piece: funny, moving, insightful, more than a little scary about being prey, almost balanced by the comfort of knowing that dogs are very loyal. Snape's innocence is convincing and heart-rending, and Sirius is deeply doglike in his human persona--they are irritating, touching, a little obnoxious, and deeply lovable. (Snape has phimosis, right? I don't suppose you'd like to write a sequel in which he gets appropriate medical treatment for it...?)