Equus caballus or the horse is a mammal. Horses, donkeys, asses, and ponies descended from a small dog-like creature called hyracotherium. It is common to think of the horse's evolution as a straight line traceable back to one species, but that is not the case. Over the eons, there were many species and sub-species each developing and evolving in response to their particular habitat.
The horse has several clues to its evolution. The chestnut, a growth of horny substance on the bone below the knee, and the ergot beneath the fetlock are the remains of toenails. The splint bones (second and fourth metatarsal in the back and metacarpal in the front) and canon (third metatarsal and metacarpal) bones were toe bones. The horse essentially stands on the tips of its fingers and toes.
Primitive horses were small, standing not much above 14 hands (56 inches / 141 centimeters). The modern horse family is divided into three classifications: heavy horses, light horses, and ponies. The size of the modern horse ranges from 5 hands (20 inches / 50 centimeters) to 19 hands (76 inches / 192 centimeters).
The modern horse is believed to be descended from four primitive horse types; a pony type thought to exist in northwest Europe, a pony thought to exist in northern Europe and Asia, a horse thought to exist in Central Asia, and a desert horse thought to have lived in Western Asia. Fossil evidence suggests that these were the ancestors of all pony and horse breeds.
Until the arrival of Spanish explorers, the horse was extinct throughout the Americas. There is fossil evidence of prehistoric horses on the western continents. Changing climate may have forced primitive horses across a land bridge to northern Asia. The wild horses now in existence such as the Mustang, or Ponies of Assateague Island are feral—having escaped from captivity and adapted to their new environment.
Mules, a cross between a male donkey (jack) and a female horse (mare), are hybrid and generally cannot reproduce. Hinnies are the offspring of a female donkey (jenny or jennet) and a male horse (stallion).
Przewalski's horses are thought to be the last truly wild horse. Other members of the Equus family are onagers, zebras, asses, and kiangs. Each of these has evolved to live in their particular environment; often, hot arid conditions that would not support a horse.
The horse is not a ruminant like cattle, which have multiple stomachs. The horse has one stomach and a long digestive tract. They are herbivores with specialized dietary requirements.
Horse’s teeth grow throughout their lives. The horse's milk teeth fall out about the age of two or three making way for adult teeth whose flat surfaces are adapted to grinding fibrous grasses and leaves.
A horse’s life span is approximately 25 years. Ponies live beyond 30 years and mules can live to their 40s.
Predator or Prey
Horses are prey animals. Their physiology and behavior are that of an animal that depends on reflexes and speed to escape predators. Their skeletons are like a human’s, but their shoulders are not anchored in a socket. This allows further reach while running.
The horse’s eyes provide almost 360-degree vision. They see well in dim light due to the tapetum lucidum, which reflects available light into the eye. They can see further than humans, although they see less color. The position of the eye provides both binocular and monocular vision. They can see forward using binocular vision. Vision to the sides and rearwards is monocular. They also have a nictitating membrane or "third eyelid," which helps protect the eye from dust and debris while grazing.
The base of a horse’s ears is very flexible. They can swivel their ears to pinpoint sounds in front and behind them. Their ears are also used to convey emotions.
Throughout their history with humans, the horse has served many purposes. The first use for horses was as food. Historians believe they were first used as draft animals. They have been used for farm work, battle, pleasure, and transport. They have pulled everything from canons to barges. They were essential transport for cowhands responsible for herding cattle over long distances and pulling omnibuses through city streets.
As the popularity and power of the internal combustion engine increased, the use of the horse shifted from work engine to pleasure animal. Today some cultures still eat horses and use them to pull loads and plows. But these practices are either frowned on or disappearing from North America. For the modern horse lover, there are hundreds of sports and activities one can enjoy with a horse or pony and many are kept just for the joy of ownership.