Since sometime before 1935, the term "Mustang" has been used by the United States Sea Services to denote enlisted men who worked their way up the ranks to officer status. Now, of course, women Marine officers with prior Marine Corps enlisted service are also Mustangs.
Going back to the Middle Ages, there was always a distinction between officers and men in the ranks which amounted to a caste system. Shakespeare in his Henry V exemplifies this when he has a sentry hail, "Art thou officer? Or art thou base, common and popular?"
In its proper sense, "Mustang" refers to the wild or half wild horse of the American plains, especially of Mexico and California, and was a distortion of the Spanish term "Mestengo".
The Detachment proudly counts two Marine Mustangs among it's members: Jim Chapin and Robert Hacker.