My favorite movie of all time. Poni beat me to it but it's loosely based on a real mountain man named John "Liver Eatin'" Johnston. There's supposed to be some truth to the situation with the Crow Indians and they sent 20 warriors after him one at a time. He killed them, cut their livers out and took a bite. Also, one Souix liver.
Many of the other characters portrayed in the movie had real life counterparts as well. Mountain men liked to build up legends about themselves as it made them more fearsome to marauding indians and murderous, theiving white men. Hatchet Jack was known for dismembering his victims with, well, a hatchet. Another guy, not in the movie, Apache Joe (I believe) supposedly had a very fine horse hair bullwhip that he could decapitate poeple with at a distance. I love mountain man lore!