I don't believe a mustang is a breed. Over the decades the breeding has been such a mix of old blood and more modern domestic breeds. Some are small, some large, all sorts of color patterns...I think of them as a mixed breed off the range. It's not that you can look at them and tell what they are because their characteristics are so varied (unless you have an isolated population that breeds to a standard more like the Kigers).
So, Mustang? Don't know...anybody's guess.
If you ever find yourself in a fair fight, it's because your tactics suck. ~ Marine 1SGT J. Reifinger