* Mare: A mare is a female horse.
* Mustang: A mustang is a feral horse, specifically descended from horses brought to the Americas by the Spanish.
So, a mustang can be a mare (if it's female), but it cannot be both at the same time.
Think of it like this:
* Mare is a biological term describing a female horse's gender.
* Mustang is a term describing a horse's origin and lifestyle (feral and descended from Spanish horses).
Just like a dog can be both a female and a golden retriever, a horse can be both a mare and a mustang.