Many of these animals were abandoned by the Spanish, or wandered off to join other wild horses. The herds flourished as they were constantly stocked with fresh introductions from runaways and abandoned horses. These horses later bred with breeds left behind by English and French explorers. Trappers, miners and cattlemen introduced new breeds in the 1800s, and farmers abandoned their horses during the Great Depression. All these factors added to the gene pool of the Western wild horse.
True mustangs are horses that have the characteristics and alleged lineage to the Spanish mustangs.
These horses are considered rare. Ironically, the wild horses tended to stay wild. Indians preferred to acquire their horses by stealing them from the Spanish settlers in the Southwest and Texas, partly because it was easier to take a ridable horse than to break a wild horse. By the mid 1700's most of the Plains Indians had converted to riding horses. Hunting became easier and contact between tribes and whites increased.