It's not in the book, nor the hanging man up the tree.
The only other Coen Brothers movie I've seen is O Brother, Where Art Thou? All the reviews I've read on the brothers seem to confer on their super cool status, in no small part due to their signature touch of nihilism. If pairing bluegrass great Ralph Stanley's "Oh Death" with the hooded Klan's man is jarring in O Brother, having Rooster Cogburn and Mattie Ross encounter the bearman who takes teeth from a corpse is absurd.
And absurdity is key. It one-ups the macabre. The brothers said in an interview that they couldn't resist putting the grown Mattie at the end of the movie (and I like that). But I think what they really couldn't resist was the insertion of the nightmarish encounter half-way in the story. The corpse-carrying medicine man is the antithesis of what Mattie Ross and Rooster stand for: a vengeance seeker and a bounty-hunter; he questions their enterprise and their purpose. The directors clearly are "enthralled" with the original story and almost fastidiously faithful to it. But I'd say the bearman is where they cut themselves some slack: stamping their own signature of absurdity.
Nihilism isn't new, but has a staying power that is irresistible, so it seems. If anyone cries foul about the romantic moonlight horse ride toward the end, he only needs to look at the hanging man swinging up-high, and the man talks in bear skin and bear speed to be pacified.
In fact I did come across a reviewer who almost mournfully complained about this most un-Coen Brother-like movie. To him, the brothers appear to have been so infatuated with the Charles Portis story that they have lost their edge, at least tentatively betrayed the artsy cult, and somehow been co-opted by the Saturday matinee crowd.
I'm no film-critic, what I have is hardly more than a hunch, which tells me that behind the super-cool armor, the directors are human after all. Armor-bearing can be exhausting in deed, and the urge of being human can be irresistible with an oblitering force. The lightness of being weighs you down, especially when scrawled and grafitted with brutality and sorrow. The Coens may question the credence in existence, yet I bet they still live as if they believed it, even if on the thin ground where they make a deity of their art. I also venture to say that in Mattie Ross and her unflinching sense of purpose and destiny, the brothers perceived, with tenderness, that it's possible to bite back the sting of death. And with that, they played themselves Rooster Cogburn for once.