How are mustangs related to domestic horses?

Mustangs are not a separate species of wild animal; rather, they are feral domestic horses that live in the wild.12

These animals are directly descended from horses that were brought to North America by the Spanish during the early colonial period and subsequently escaped or were released. Because they are descendants of domesticated stock that have returned to a wild state, they are classified as feral rather than truly undomesticated wild horses, such as Przewalski’s horse.321

In summary, mustangs are biologically the same as other members of the species Equus caballus, sharing the same ancestry as the domestic horses kept by humans today.2