Trivia: Part of the reason Alice (the survivor of the first film) is only on screen during the first scene and is killed off early is because the actress portraying her (Adrienne King) was dealing with a real-life stalker and was trying to limit her acting appearances.
Trivia: An actor/stuntman named Steve Dash actually played Jason in almost every scene, but was crestfallen when he found out he wouldn't be given on-screen credit for the role. Due to some studio red-tape, the actor credited for portraying Jason was Warrington Gillette, a stand-in who only appeared on-screen for a brief instance. (Gillette portrayed the "Unmasked Jason" under heavy makeup, who leaps through the window at the end of the film and is only on-screen for about 20 seconds.) Dash was understandably upset, as he put in almost all the work to create and play the character, but got no reward and was only credited as being Jason's stunt-double. When archival footage from this film was re-used in the next two installments in flashback, Dash was finally given the proper credit he deserved and was given sole credit for portraying Jason via archival footage.
Trivia: The movie ends with a shot of Mrs. Voorhees old wrinkled head on Jason's shrine. Originally her head was supposed to open its eyes, and smile. Steve Miner comments on commentary how he decided to get rid of the idea, as he felt it would make the movie come across as more cheap and hokey than normal.
Trivia: The working title for the film was simply "Jason," before the producers decided to call it "Friday the 13th Part 2."
Trivia: At different times in the film, we see Jason carrying a pitchfork and a pickaxe, but he never actually kills anyone using either of these two weapons, at least not in this entry.
Trivia: There's a very fun, subtle "blink and you'll miss it" detail in the climax. When Ginny finds the head of Mrs. Voorhees in the shack, in the first shot, you can actually briefly see the decomposing (almost mummified) body of Alice, the only survivor of the original film, on the floor. What gives it away? The ice-pick Jason stabbed her with at the beginning of this movie is still sticking out of her forehead.
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.