The idea that prehistoric hunters, wielding the big spear points associated with Folsom Man, etc., killed all much of the North American megafauna (mammoths, giant ground sloths, etc.) has become almost "gospel." Likewise, these hunters are accused of killing off the former giant bison and/or forcing them into dwarfism--yes, the bison or American buffalo we know now would be the "dwarf."
New research, however, suggests that the hunting peoples maybe were not to blame.
On that larger topic of "missing" animals, Connie Barlow's Ghosts of Evolution is an excellent read. After reading it last year, I can never look at an avocado or the common roadside "coyote gourd" the same away again.