The Spanish introduced horses to North America in the first or second decade of the 1500s, and the first native American tribes acquired horses via trade (rather than capture of wild horses) so they were able to view already established methods of horsemanship and learn those skills rather quickly. So you could say the development of horsemanship among plains Native Americans it was more of a technology transfer than re-inventing the wheel. And given the game-changing nature of the horse (allowed for more and better hunting, huge warfare advantage), the tribes who couldn't master the horse were most likely defeated, driven out, eradicated, or assimilated by those who could, thus ensuring in a relatively short amount of time that the North American plains would be inhabited by people who were masters of horsemanship. By the time Lewis and Clark arrived on the scene, tribes like the Sioux and the Commanche have already mastered the horse for over 2 centuries.