A wendigo. I can't believe nobody put this together. He ate his friend's finger, then he moved into a cave, then his skin stretched over his bones, then his teeth and fingernails grew out, and he eats people.
[spoiler][b]OPEN YOUR EYES PEOPLE[/b][/spoiler]
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Not to stomp on your parade but when J.R.R. Tolkien wrote the book(s) he was referring to his own mind for reference. All characters are completely made up from his kind only. Gollum is, well.. Gollum.