Is it ensure, insure, or assure?
"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."
A commissioned officer who has been promoted from the enlisted ranks: A mustang who had worked his way up from the ranks in 13 years
[1847+ Armed forces; fr mustang, ''wild, sturdy horse of the Western US,'' fr Mexican Spanish mestengo, ''stray animal'']
North American wild or Indian-tamed horse, descended from horses taken to the New World by the Spanish in the 16th century. The name comes from that of an Indian tribe of eastern Washington and Oregon that was known for the small horses it bred.