A wild horse.
1808. Passed several herds of mustangs or wild horses.Z. M. Pike, Sources of the Mississippi, iii. 273. (N.E.D.)
1822. The inhabitants of many places [in Texas] were subsisting on the flesh of mustangs (wild horses), and even that was scarce.Mass. Spy, Oct. 9.
1834. Lewis and Irwin obtained young and unbroken wild horses (or as the hunters call them, mestangs.)Albert Pike, Sketches, &c. p. 74 (Boston).
1844. A hardy, sensible mustang, who dated his origin from the plains of Arkansas.Yale Lit. Mag., ix. 262 (April). (Italics in the original.)