Merriam-Webster Collegiate Dictionary - mustang

 noun  Etymology: Mexican Spanish mestengo, from Spanish, stray, from mesteno strayed, from mesta annual roundup of cattle that disposed of strays, from Medieval Latin (animalia) mixta mixed animals  Date: 1808  1. a small hardy naturalized horse of United States western plains directly descended from horses brought in by the Spaniards; also bronc  2. slang a commissioned officer (as in the United States Navy) who has risen from the ranks