The American Indian Horse is any horse of Spanish origin that has evolved to adapt to a particular environment within the United States, with or without breeding from humans. The title American Indian Horse does not refer to one specific breed; rather, it applies to any breed that has proved itself capable of withstanding a distinct ecotone, whether it be the high plains of the Midwest or the low swamplands of the South.

http://en.wikipedia.org/wiki/American_Indian_Horse