In the Navy, a Mustang is an Officer who has promoted up from the ranks of Navy enlisted personnel through an in-service procurement program with no interruption of his/her active duty status, hence the motto "sursum ab ordine" which is Latin for "up from the ranks". It is also understood that the Mustang Officer was a career Sailor with one or more service stripes which indicate at least four years of enlisted service and qualifiy the prior enlisted to be commissioned in the paygrade of O-1E vice O-1.
The Navy Mustang's commissioning source is through one of the following programs: Limited Duty Officer (LDO), Chief Warrant Officer (CWO), Fleet Accession to Officer Candidate School (OCS), Medical Enlisted Commissioning Program (MECP) or, as in my case, Seaman to Admiral - 21st Century (STA-21). In the past, there were other programs leading to a commission such as the Enlisted Commissioning Program (ECP), the Aviation Enlisted Commissioning Program (AECP), the Nuclear Enlisted Commissioning Program (NECP), the Civil Engineer Corps Enlisted Commissioning Program (CECECP), Fleet Accession to the Naval Reserve Officers Training Corps (NROTC), and Broadened Opportunity for Officer Selection and Training (BOOST); but, these have since gone away.
The term "Mustang" is a relatively modern term, originating either just prior to or during World War II. It is believed to be a Sea Service term, although other service officers are beginning to be described as Mustangs.
The title literally refers to the mustang horse which is a wild animal, not a thoroughbred. A mustang, after being captured, can be tamed and saddle broken, but it always has a bit of a wild streak and can periodically revert to its old ways unexpectedly. Therefore, the owner must keep an eye on it at all times.
By the same token, however, since a mustang was formerly a wild and free animal, it may very well be smarter, more capable and have a better survival instinct than thoroughbreds. The mustang can take care of itself when the going gets tough, thriving on rough treatment, while the thoroughbred, having been pampered its whole life, cannot.
So, you can easily see the parallel between horses and Naval Officers. The term "Mustang" is used in a complimentary sense, most of the time.
Another accounting for the nickname "Mustang" is derived from the U.S. Naval Academy where it was initially used by USNA grads as an insult to the "lower class" of prior enlisted officers. They considered themselves to be thoroughbreds and, meaning to insult, called our honored predecessors "Mustangs" because we were less than pure breeds in their eyes. Like "Yankee", the slur stuck and soon became a badge of honor.
Semper Fortis
