The creature was basically a Silverback Gorilla....
The creatures size, posture, strength, intelligence, and the way it moved was pretty much a gorilla. Other than it being hairless with reptilian skin and a narly set of teeth.
I'm thinking maybe the movie would have been cooler if it had turned out to be a large silverback gorilla that somehow, someway ended up in those woods and has had to fend for himself for a long time. Lord knows how many wolves and bears he's had to deal with.
This would also better explain the creature's behavior towards humans. When the girl was right beside it why did he not kill her? If the creature were a gorilla that had intelligence and perhaps at some point in the past had human handlers than that would explain the curiosity the creature exhibited, not just a blind killing machine.
Ripping off the man's arm and tossing it on the car.....as if it were playing. Allowing the guy to live and crawl to his car only to decide to kill him.
Especially at the end in the ambulance when he was thrashing the inside frustrated looking for the girl, I mean that was a gorilla all day. And earlier in the film when it was attacking her mom and she scared it away by screaming at it. Obviously he didn't really have anything to fear from the girl, but perhaps some kind of long gone submissive training from a human was triggered and that's what made it run off.
Anyway the movie seemed very grounded in reality so I think it would have been really cool to go the way of Cujo and had a "monster" that could be of this Earth but every bit as terrifying as something we could make up in our heads. Would have improved the movie's score in my opinion.