Equus—the genus to which all modern equines, including horses, asses, and zebras, belong—evolved from Pliohippus some 4 million to 4.5 million years ago during the Pliocene. Equus shows even greater development of the spring mechanism in the foot and exhibits straighter and longer cheek teeth. Mustangs are often referred to as wild horses, but because they are descended from once-domesticated animals, they are actually feral horses. The original mustangs were Colonial Spanish horses, but many other breeds and types of horses contributed to the modern mustang, now resulting in varying phenotypes. The Appaloosa is a horse breed associated historically with the Nez Perce (Nimiipuu) Tribe. The name may originate from “a Palouse,” which referred to the region where the horses were bred.