mustang

mustangi A small, hardy, naturalized (feral) horse of the North American west.
1846, w:Thomas Bangs Thorpe|Thomas Bangs Thorpe, The Mysteries of the Backwoods, Carey and Hart, page http://books.google.com/books?id=k1ITAAAAYAAJ&pg=PA12&dq=mustang+%22invariable+companion%22+narrow-chested&lr=lang_en&num=50&as_brr=0&as_pt=ALLTYPES&ei=05TPSZjpOomGNsSKnBEPPA12,M1 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.

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

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

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.

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

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