Horse
The horse adapted to survive in areas of wide-open terrain with sparse vegetation, surviving in an ecosystem where other large grazing animals, especially ruminants, could not. Horses and other equids are odd-toed ungulates of the order Perissodactyla, a group of mammals that was dominant during the Tertiary period. In the past, this order contained 14 families, but only three—Equidae (the horse and related species), Tapiridae (the tapir), and Rhinocerotidae (the rhinoceroses)—have survived to the present day.
Equus (genus)
The genus "Equus" was first described by Carl Linnaeus in 1758. It is the only recognized extant genus in the family Equidae. The first equids were small, dog-sized mammals (e.g. "Eohippus") adapted for browsing on shrubs during the Eocene, around 54 million years ago (Mya). These animals had three toes on the hind feet and four on the front feet with small hooves in place of claws, but also had soft pads. Equids developed into larger, three-toed animals (e.g. "Mesohippus") during the Oligocene and Miocene. From there, the side toes became progressively smaller through the Pleistocene until the emergence of the single-toed "Equus".