Equus caballus caballus, the modern horse, in the Equidae family of the Perissodactyls (odd-toed ungulates) evolved from three primitive ancestors in Eurasia by the end of the last Ice Age, roughly 10,000 years ago. From their early uses in carrying vehicles and pulling loads, horses have been selected into hundreds of specialized breeds used around the world for transportation (either ridden or harnessed to convey wheeled vehicles), agriculture (to pull equipment and ride after cattle), and war (for carrying troops into battle and carting equipment), as well as in numerous competition and show disciplines, ranging from racing to Olympic sports (dressage, jumping, and eventing—a 3-part competition that includes dressage, cross-country, and show jumping) to various Western and rodeo events, including reining, barrel-racing, and team penning, as well as in therapy work. Most horses are domesticated, but there are some feral populations in isolated pockets around the world.

Horses are medium to large mammals, with long heads and necks with a mane. Their legs are slender and end in a single toe, protected by a horny hoof. They have long, slender tails, either ending in a tuft, or entirely covered in flowing hair. They are adapted to generally open terrain, from plains and savannas, to mountains or deserts. Equines are one of only two mammals (the other is the human) capable of producing copious sweat perspiration for thermoregulatory cooling, enabling fast running over long distances.

Horses are herbivores, and feed predominantly on tough, fibrous food, such as grasses and sedges. When in need, they will also eat other vegetable matter, such as leaves, fruits, or bark, but are normally grazers, not browsers. Unlike ruminants, with their complex stomachs, equines break down cellulose in the "hindgut" or caecum, a part of the colon.

Equus caballus sylvaticus, the Forest or Diluvian Horse, appears to be the ancestor of the modern draft and heavyweight breeds; these horses were slow-moving and stoutly built, with coarse, thick coats, manes, and tails. Equus caballus gmelini Antonius, the Tarpan horse, from eastern Europe and the steppes of southern Russia, had a smaller, lighter build, but was hardy, strong and speedy; many pony and light horse breeds are believed to have evolved from this lineage. Equus caballus przewalskii prsewalskii Poliakov, the Asiatic horse, was discovered in the wild in Mongolia in 1879, and a few populations still live in the wild in eastern Asia, as well as in zoos. Although the Asiatic horse, which is hardy, cold-resistant and dun with darker mane and tail, is considered to be ancestral to many breeds, it has a different chromosome number (33, vs. the 32 found in modern horse breeds). Differing lineages or combinations of the three original subspecies in turn yielded four general types of ponies and horses from which all modern breeds derive.

The horse was first domesticated 5,000 to 6,000 years ago, perhaps earlier. Horses appear to have been domesticated in conjunction with the evolution of agriculture. From this start, horse breeds were developed and used in warfare and for transportation.

(McBane 2005, Wikipedia 2012.)