"small, half-wild horse of the American prairie and pampas," 1808, from Mexican Spanish mestengo "animal that strays" (16c.), from Spanish mestengo "wild, stray, ownerless," literally "belonging to the mesta," an association of cattle ranchers who divided stray or unclaimed animals that got "mixed" with the herds, from Latin mixta "mixed," fem. past participle of miscere "to mix" (from PIE root *meik- "to mix").
Said to be influenced by the Spanish word mostrenco "straying, wild," which is probably from mostrar, from Latin monstrare "to show." Though now feral, the animals are descended from tame horses brought to the Americas by the Spaniards. The brand of automobile was introduced by Ford in 1962.