Literature
Are Mustangs Native, Exotic, or Neither?
Before I begin, there are a few things you must understand:
Equus is the equine genus. It contains all species and subspecies of equines, including, but not limited to, horses, asses, zebras, etc.
Equus ferus is the horse species. It's the "wild type" horse, although not all of its subspecies are wild, of course. It contains all modern subspecies of horses. en.wikipedia.org/wiki/Wild_hor…
Equus ferus ferus is a subspecies of the above Equus ferus species, and was the true wild horse. It was known as the Tarpan and is now extinct. petermaas.nl/extinct/speciesin… , www.departments.bucknell.edu/b… , en.wikipedia.org/wiki/Tar