mustangyoudaoicibaDictYouDict[mustang 词源字典]
mustang: [19] Etymologically, a mustang is a ‘mixed’ animal. The word comes from Mexican Spanish mestengo, which originally in Spanish meant ‘stray’. This was derived from mesta ‘annual roundup of cattle, participated in by all the herdsmen, in which stray cattle were disposed of’, which in turn goes back to medieval Latin mixta. And mixta (literally ‘mixed’) was used for the wild or stray animals that got ‘mixed’ in with the graziers’ herds (it was a noun use of the feminine past participle of miscēre ‘mix’, source of English miscellaneous and mix).

The word passed early on from ‘stray cattle’ to ‘stray horses’.
=> miscellaneous, mix[mustang etymology, mustang origin, 英语词源]
mustang (n.)youdaoicibaDictYouDict
"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."