In the seventeenth century, horses were brought to the Great Plains by the
Spanish, and the arrival of these previously unknown animals
revolutionized the lives of the Plains Indians.  Horses were a valuable
asset for hunting bison; not only was their speed an advantage, but their
height improved the accuracy of archers and spear throwers.  Later, when
guns were introduced, the Indians became expert at mounted warfare. 
Horses also served as beasts of burden, transporting goods in travoises as
these nomadic tribes migrated with the seasons.  The wild mustang evolved
from the first horses that came to the Plains; they developed strength and
intelligence which enabled them to thrive in a frequently harsh
environment.