From Cheyenne Autumn:
The man riding the horse is Harry Carey, Jr.
The man leaping onto the horse is a young Dean Smith, doubling for Ben Johnson, who had was unable to do the stunt because he had broken his foot when he dropped something on it (I forget exactly what he had dropped but it was heavy).
Dean Smith's leap would make Mikhail Baryshnikov himself jealous, he achieves it with such grace, beautiful form, and accuracy.
