Character mistake: Ginny should've been loaded head first into the ambulance, not feet first.
Other mistake: The narration in the film's trailer incorrectly states, "On a June night in 1980, Friday the 13th, twelve of her friends were murdered." In reality, only eight people were killed then, including the killer Mrs. Voorhees, whom Alice herself killed in self-defense. Mrs. Voorhees was not Alice's friend.
Continuity mistake: When Scott gets his throat cut, blood pours out. Yet when Terri spins him around, there's no blood anywhere on him except a small amount on his shirt sleeves.
Other mistake: Right at the part where Crazy Ralph is looking in on Paul and Ginny. Ralph hears a breaking of a stick behind him. You can see Jason put the barbed wire up and over the tree. How is that possible? He would have to whip it around the tree, not over it. (00:30:20)
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)
Revealing mistake: At the end of the film when Ginny and Paul are fighting Jason, his machete hits the shrine where his mother's head is displayed and where Terri's body is strewn. When the machete hits, Terri flinches, even though she is dead. (01:16:00)
Continuity mistake: When Scott is dancing with the dog, in the shots from inside the cabin, the collar on his shirt is up. But from the shots outside the cabin looking in, his collar is down. (00:26:50)
Continuity mistake: When the girl who owned the dog is hit in the bum with a slingshot, she is hit in the right butt cheek, yet she grabs her left butt cheek. (00:19:00)
Continuity mistake: When the girl runs to the car to escape Jason, the wipers are in a position different from when the car had been turned off. (01:03:20 - 01:07:25)
Continuity mistake: As their car is towed, the guy hangs up the phone and it remains on the hook. When it cuts the phone is hanging off the hook, as they are running from the phone booth. (00:15:00)
Continuity mistake: When Ginny is in the VW, she knocks Jason over with the car door. After, Jason gets up twice. (01:08:15)
Revealing mistake: When Ginny has run out to her VW Bug and is attempting to start it, Jason is supposed to be back in the house just on his way out to get her. But if you look at Ginny sitting in the car, in the lefthand corner of the windshield you can see a reflection of the top of Jason's hood. He was there all the time, crouched down beside the fender just waiting for his cue to pop up. (01:07:45)
Continuity mistake: When Ginny puts on her red robe and fixes her hair in front of the mirror, some of her fringe is sticking out. When it cuts her hair is hanging down perfectly. (00:28:35)
Audio problem: When the band starts to play the guitar solo doesn't match the guitar player's fingers or picking. (00:41:10)
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)
Continuity mistake: When Jason enters Ginny's cabin, all that is shown are his feet. After running through rain and mud, he leaves no tracks whatsoever, and his shoes are spotless. (01:10:20)
Continuity mistake: The photo pinned beside the window changes in appearance and position during shots. (01:18:45)
Continuity mistake: When Terri is undressing to go skinny-dipping, she changes out of her clothes and her hair is dry. In the very next shot, she is entering the water and her hair is moist and slicked back. (00:44:25)
Continuity mistake: When Mark is on the porch, the camera is Jason's point of view and Jason is sneaking up behind Mark. However, then the camera switches to Mark's face and there is no one behind him. (00:58:05)
Continuity mistake: When Terri is walking back to the cabin to get a knife, her pants are untied at the waist. When she reaches the cabin, they are tied with a bow, yet when walks back to Scott, they are untied again. (00:47:45)
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.