'Untamed': Wild Mustangs Roam the West

This stunning vista is the Big Horn Canyon in Montana's Pryor Mountains, home to about 175 wild mustangs. After National Geographic captured a rare glimpse of how these untamed animals exert their dominance in the wild West, "Nightline" anchor Bill Weir traveled to the Montana-Wyoming border to see the horses up-close. (Bill Weir/ABC)