Visible crew/equipment: As the camera moves in on the shower curtain when Alice is taking a shower, you can see the shadow of the camera. (00:08:35)
Visible crew/equipment: When Ginny and Paul walk down the stairs after they found the bed drenched in blood, Paul walks by the camera and you can see the camera's shadow on his chest. You can tell it's the camera's shadow, because when it cuts Ginny walks the same way and no shadow falls on her chest. (01:04:35)
Visible crew/equipment: When Jason is in Ginny's cabin, you can see the mike wire's shadow on the floor when he starts walking towards the cots. (01:10:30)
Visible crew/equipment: When Terri is back at the cabin looking for a knife, she crouches down giving us a clear view of the mirror. Look closely, you can see a crew member carefully moving in the right side of the mirror. (00:48:00)
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.