Etymology

Noun

1. A small, hardy, naturalized (feral) horse of the North American west.
   • 1846, Thomas Bangs Thorpe, The Mysteries of the Backwoods, Carey and Hart, page 12:

     The mustang pony, the invariable companion of the inhabitant of the prairie, whether he is rich or poor, is a little creature, apparently narrow-chested, and small across the loins.

   • 1851, Mayne Reid, The Scalp Hunters; Or, Romantic Adventures in Northern Mexico, vol. 3, Charles J. Skeet, page 145:

     Having ridden a distance of two or three miles, Garey slackened his pace, and put the mustang to a slow walk.

2. A merchant marine who joined the U.S. Navy as a commissioned officer during the American Civil War.
   • 1903, James Hoyt (editor), Seen & Heard by Megargee, L.N. Megargee, page 1754:

     He is a product of the merchant marine and is one of the officers called "Mustangs" who entered the navy during the Civil War.

   • 1939, Fred J. Buenzle, Bluejacket, W. W. Norton & Company, page 179:

     He was the son of a famous artist, and was what we termed a "mustang" officer, who had come into the navy from the merchant service during the Civil War.

3. A commissioned officer who started military service as an enlisted person.
   • 1918, Alfred Emanuel Smith, New Outlook, volume 120 (September–December 1918), Outlook Publishing Company, page 417:

     … and the chief engineer is a "mustang" — that is, an officer who has risen from the ranks of enlisted men.

   • 1943, Josef Israels, He's in the Marine Corps Now, R.M. McBride & Company, page 170:

     Mustang— Officer who came up through the ranks. None better.