What makes a horse a "mustang" really?

Yeah I know its a wild horse but does it have to have been wild to be a mustang or had wild ancestors? I know if you say get a mustang mare and stallion and you bred them a few generations they would still be a mustang because there parents were even though they were never wild themselves,so what if there was a horse...lets say a paint who when it grew up had like all the characteristics of a mustang like the sturdiness,the heavy mane,the pony size,and shorter stature...you know what I am saying? so what are your thoughts on this?