The major impact of the coming of horses was to completely change the way of life of the Plains Indians.
When horses came, the Indians came to be able to make their living from buffalo much more easily. They were able to follow the buffalo herds and to bring many more things with them as they did so. This allowed them to have a higher standard of living than they previously had.
The coming of horses also changed warfare. Those tribes who were quickest to realize the ways in which horses could be used in war came to dominate.
In these ways, the coming of horses changed human life on the Great Plains, creating the Plains Indians who are such a big part of our image of the 1800s.