The horse is a domesticated hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of Equus ferus. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature, Eohippus, to today's large, single-toed animal. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have spread widely by 3000 BC. Horses are domesticated in the Cabalus subspecies, although some domesticated populations live in the wild as feral horses. These feral groups are not true wild horses, as this term is used to describe horses that were never domesticated. There is an extensive specialized vocabulary used to describe concepts related to horses, covering everything from anatomy to life stages, size, colours, markings, breeds, movement, and behavior.

Horses are adapted for running, which allows them to quickly escape from predators, have an excellent sense of balance and a strong fight-or-flight response. Related to this need to escape predators in the wild is an unusual trait: horses are able to sleep standing and lying down, with younger horses tending to sleep significantly more than adults. Female horses, called mares, carry their young for about 11 months, and a young horse, called a foal, can stand and run soon after birth. Most domesticated horses begin training bareback or in harness between the ages of two and four. They reach full adult growth by the age of five, and have an average lifespan of between 25 and 30 years.