The horse (Equus ferus caballus) is a hooved (ungulate) mammal, a subspecies of the family Equidae. The horse has evolved over the past 45 to 55 million years from a small multi-toed creature into the large, single-toed animal of today. Humans began to domesticate horses around 4000 BC, and their domestication is believed to have been widespread by 3000 BC. Although most horses today are domesticated, there are still endangered populations of the Przewalski's Horse, the only remaining true wild horse, as well as more common populations of feral horses which live in the wild but are descended from domesticated ancestors. There is an extensive, specialized vocabulary used to describe equine-related concepts, covering everything from anatomy to life stages, size, colors, markings, breeds, locomotion, and behaviour.

Horses' anatomy enables them to make use of speed to escape predators and they have a well-developed sense of balance and a strong fight-or-flight instinct. Related to this need to flee from predators in the wild is an unusual trait: horses are able to sleep both standing up and lying down. Female horses, called mares, carry their young for approximately 11 months, and a young horse, called a foal, can stand and run shortly following birth. Most domesticated horses begin training under saddle or in harness between the ages of two and four. They reach full adult development by age five, and have an average lifespan of between 25 and 30 years.

Horse breeds are loosely divided into three categories based on general temperament: spirited "hot bloods" with speed and endurance; "cold bloods", such as draft horses and some ponies, suitable for slow, heavy work; and "warmbloods", developed from crosses between hot bloods and cold bloods, often focusing on creating breeds for specific riding purposes, particularly in Europe. There are over 300 breeds of horses in the world today, developed for many different uses.

Horses and humans interact in a wide variety of sport competitions and non-competitive recreational pursuits, as well as in working activities such as police work, agriculture, entertainment, and therapy. Horses were historically used in warfare, from which a wide variety of riding and driving techniques developed, using many different styles of equipment and methods of control. Many products are derived from horses, including meat, milk, hide, hair, bone, and pharmaceuticals extracted from the urine of pregnant mares. Humans provide domesticated horses with food, water and shelter, as well as attention from specialists such as veterinarians and farriers.

Horse anatomy is described by a large number of specific terms, as illustrated by the chart to the right. Specific terms also describe various ages, colors and breeds.

Lifespan and life stagesDepending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years. It is uncommon, but a few animals live into their 40s and, occasionally, beyond. The oldest verifiable record was "Old Billy", a 19th-century horse that lived to the age of 62. In modern times, Sugar Puff, who had been listed in the Guinness Book of World Records as the world's oldest living pony, died in 2007, aged 56.

Regardless of a horse's actual birth date, for most competition purposes an animal is considered a year older on January 1 of each year in the northern hemisphere and August 1 in the southern hemisphere. The exception is in endurance riding, where the minimum age to compete is based on the animal's calendar age. A very rough estimate of a horse's age can be made from looking at its teeth.

The following terminology is used to describe horses of various ages:

Foal: a horse of either sex less than one year old. A nursing foal is sometimes called a suckling and a foal that has been weaned is called a weanling. Most domesticated foals are weaned at 5 to 7 months of age, although foals can be weaned at 4 months with no adverse effects.
Yearling: a horse of either sex that is between one and two years old.
Colt: a male horse under the age of four. A common terminology error is to call any young horse a "colt", when the term actually only refers to young male horses.
Filly: a female horse under the age of four.
Mare: a female horse four years old and older.
Stallion: a non-castrated male horse four years old and older. Some people, particularly in the UK, refer to a stallion as a "horse".
Gelding: a castrated male horse of any age.
In horse racing, these definitions may differ: For example, in the British Isles, Thoroughbred horse racing defines colts and fillies as less than five years old. However, for Australian Thoroughbred racing, colts and fillies are less than four years old.

Modern horses are often used to reenact many of their historical work purposes. Horses are used, complete with equipment that is authentic or a meticulously recreated replica, in various live action historical reenactments of specific periods of history, especially recreations of famous battles. Horses are also used to preserve cultural traditions and for ceremonial purposes. Countries such as the United Kingdom still use horse-drawn carriages to convey royalty and other VIPs to and from certain culturally significant events. Public exhibitions are another example, such as the Budweiser Clydesdales, seen in parades and other public settings, a team of draft horses that pull a beer wagon similar to that used before the invention of the modern motorized truck.

Horses are frequently seen in television and films. They are used both as main characters, in films such as Seabiscuit, and Dreamer, and as visual elements that assure the accuracy of historical stories. Both live horses and iconic images of horses are used in advertising to promote a variety of products. The horse frequently appears in coats of arms in heraldry. The horse can be represented as standing, walking (passant), trotting, running (courant), rearing (rampant or forcine) or springing (salient). The horse may be saddled and bridled, harnessed, or without any apparel whatsoever. The horse also appears in the 12-year cycle of animals in the Chinese zodiac related to the Chinese calendar. According to Chinese folklore, each animal is associated with certain personality traits, and those born in the year of the horse are intelligent, independent, and free-spirited.