Character mistake: Ginny should've been loaded head first into the ambulance, not feet first.
Character mistake: Ginny is supposed to be put head first into the ambulance, not feet first. All the monitoring equipment is in the front of the head. (01:24:20)
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.