"small, half-wild horse of the American prairie," 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" (see mix (v.)).
Said to be influenced by the Spanish word mostrenco "straying, wild," which is probably from mostrar, from Latin monstrare "to show."
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