
Revealing mistake: Just after the scene where Mark is killed, there's a shot of Jason's feet as he opens a screen door and walks across a dusty hardwood floor. As the dolly pulls backward across the floor, you can see the camera's roller marks in the dust. (00:59:10)
Answer: It never really says what the body is, but I'd assume it were a rabbit or some other small woodland creature. It can't be Muffin because, like you said, Muffin appears later on.