The horse (Equus caballus) is a large odd-toed ungulate mammal, one of ten living species of the family Equidae.

For centuries horses have been one of the most economically important domesticated animals, especially relied upon for farmwork and for transportation. Their importance declined following the introduction of mechanization. The history of the horse is prominent in religion, mythology, art, transportation, agriculture, and warfare.

Most horses perform work such as carrying humans or are harnessed to pull objects such as carts or plows. Hundreds of distinct horse breeds have been developed, allowing horses to be specialized for certain tasks; lighter horses for racing or riding, heavier horses for farming and other tasks requiring pulling power. Some horses, such as the miniature horse, can be kept as pets. In some societies, horses are a source of food, both meat and milk; in others it is taboo to consume these products. In industrialized countries, horses are predominantly kept for leisure and sporting pursuits, while in other parts of the world they are still used as working animals.

Horses and humans have lived and worked together for thousands of years and an extensive specialized vocabulary has arisen to describe virtually every horse behavioral and anatomical characteristic with a high degree of precision.

Horses exhibit a diverse array of coat colors and distinctive markings, and a specialized vocabulary has evolved to describe them. Often, one will refer to a horse in the field by its coat color first rather than by breed or by sex. In spite of the adage that "a good horse is never a bad color," flashy or unusual colors are sometimes very popular, as are horses with particularly attractive markings. Horses of the same color may be distinguished from one another by their markings.

The genetics of horse coat colors has largely been mapped, although research continues to be conducted on the identification of specific genes and mutations that result in specific color traits. Essentially, all horse colors begin with a genetic base of "red" (chestnut) or "black," with the addition of alleles for suppression of color, dilution of color, spotting, graying, or other effects acting upon the base colors in various combinations and varying degrees of dominance or recessivity that create the dozens of possible shades of horses.

Horses that are light in color are often misnamed as being "white" horses. A horse that looks pure white is, in most cases, actually a middle-aged or older gray. Grays have black skin underneath their white hair coat (with the exception of small amounts of pink skin under white markings). This is how a gray horse can be distinguished from a white horse. The only horses properly called white are those with pink skin under a white hair coat, a far more rare occurrence. There are no truly albino horses (white skin and pink eyes). True albinism is a lethal gene in horses.

Pregnancy lasts for approximately 335-340 days and usually results in one foal. Twins are rare. Colts are carried on average about 4 days longer than fillies. Horses, particularly colts, may sometimes be physically capable of reproduction at approximately 18 months but in practice are rarely allowed to breed until a minimum age of 3 years, especially females. Horses four years old are considered mature, though the skeleton usually finishes developing at the age of six, and the precise time of completion of development also depends on the horse's size (therefore a connection to breed exists), gender, and the quality of care provided by its owner. Also, if the horse is larger, its bones are larger; therefore, not only do the bones take longer to actually form bone tissue (bones are made of cartilage in earlier stages of bone formation), but the epiphyseal plates (plates that fuse a bone into one piece by connecting the bone shaft to the bone ends) are also larger and take longer to convert from cartilage to bone as well. These plates convert after the other parts of the bones do but are crucial to development.

Depending on maturity, breed and the tasks expected, young horses are usually put under saddle and trained to be ridden between the ages of two and four. Although Thoroughbred race horses are put on the track at as young as two years old in some countries, horses specifically bred for sports such as dressage are generally not entered into top-level competition until a minimum age of four years old, because their bones and muscles are not solidly developed, nor is their advanced training complete. For endurance riding competition, horses may not compete until they are a full 60 calendar months (5 years) old.

The English-speaking world measures the height of horses in hands, abbreviated "h" or "hh," and is measured at the highest point of an animal's withers. One hand is 4 inches (10 cm). Intermediate heights are defined by hands and inches, rounding to the lower measurement in hands, followed by a decimal point and the number of additional inches between 1 and 3. Thus a horse described as 15.2 hh tall, means it is 15 hands, 2 inches, or 62 inches (160 cm) in height.

The size of horses varies by breed, but can also be influenced by nutrition. The general rule for cutoff in height between what is considered a horse and a pony at maturity is 14.2 hands (58 inches (150 cm)) as measured at the withers. An animal 14.2h or over is usually considered a horse and one less than 14.2h is a pony. However, there are exceptions to the general rule. Some smaller horse breeds who typically produce individual horses both under and over 14.2h are considered "horses" regardless of height. Likewise, some pony breeds, such as the Welsh pony, share some features of horses and individual animals may occasionally mature at over 14.2h, but are still considered ponies.

The difference between a horse and pony is not simply a height difference, but also a difference in phenotype or appearance. There are noticeable differences in conformation and temperament. Ponies often exhibit thicker manes, tails and overall coat. They also have proportionally shorter legs, wider barrels, heavier bone, shorter and thicker necks, and short heads with broad foreheads. They often have calmer temperaments than horses and also a high level of equine intelligence that may or may not be used to cooperate with human handlers.

Light riding horses such as Arabians, Morgans, or Quarter Horses usually range in height from 14.0 (56 inches (140 cm)) to 16.0 hands (64 inches (160 cm)) and can weigh from 850 pounds (390 kg) to 1,200 pounds (540 kg). Larger riding horses such as Thoroughbreds, American Saddlebreds or Warmbloods usually start at about 15.2 hands (62 inches (160 cm)) and often are as tall as 17 hands (68 inches (170 cm)), weighing from 1,100 pounds (500 kg) to 1,500 pounds (680 kg). Heavy or draft horses such as the Clydesdale, Belgian, Percheron, and Shire are usually at least 16.0 (64 inches (160 cm)) to 18.0 hands (72 inches (180 cm)) high and can weigh from about 1,500 pounds (680 kg) to 2,000 pounds (910 kg). While ponies cannot be taller than 14.2h (147 cm), they may be much smaller, down to the Shetland pony at around 10 hands (40 inches (100 cm)), and the Falabella which can be no taller than 30 inches (76 cm), the size of a medium-sized dog. However, while many miniature horse breeds are small as or smaller than a Shetland pony, because they are bred to have a horse phenotype (appearance), their breeders and registries classify them as very small horses rather than ponies.

It is thought that the largest horse in (recorded) history was a Shire horse named Sampson, who lived during the late 1800s. He stood 21.2½ hands high (86.5 inches (220 cm)), and his peak weight was estimated at 3,360 pounds (1,520 kg). The current record holder for the world's smallest horse is Thumbelina, a fully mature miniature horse affected by dwarfism. She is 17 inches (43 cm) tall and weighs 60 pounds (27 kg).

Depending on breed, management and environment, the domestic horse today has a life expectancy of 25 to 30 years. It is uncommon, but a few horses live into their 40s, and, occasionally, beyond. The oldest verifiable record was "Old Billy," a horse that lived in the 19th century 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 then-living pony, died at age 56.

Regardless of a horse's actual birthdate, for most competition purposes, horses are considered a year older on January 1 of each year in the northern hemisphere and August 1 in the southern hemisphere. The exception is endurance riding, where the minimum age to compete is based on the horse's actual 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 4-6 months of age.

* 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." A Ridgling or "Rig" is a stallion which has an undescended testicle. If both testicles are not descended, the horse may appear to be a gelding, but will still behave like a stallion.

* Gelding: a castrated male horse of any age, though for convenience sake, many people also refer to a young gelding under the age of four as a "colt."

In horse racing, the definitions of colt, filly, mare, and stallion or horse may differ from those given above. In the United Kingdom, Thoroughbred horse racing defines a colt as a male horse less than five years old, and a filly as a female horse less than five years old. In the USA, both Thoroughbred racing and harness racing defines colts and fillies as four years old and younger.

Horse breeds are groups of horses with distinctive characteristics, such as conformation, color, performance, intelligence or disposition, that are transmitted consistently to their offspring. These inherited traits are usually the result of a combination of natural crosses and artificial selection methods aimed at developing horses for specific tasks. Certain breeds are known for certain talents, for example, Standardbreds are known for their speed in harness. Some breeds have been developed through centuries of crossings with other breeds, while others such as Tennessee Walking Horses and Morgans developed from a single sire with a tremendous influence. There are more than 300 horse breeds in the world today.

Different schools of thought exist to explain how this range of size and shape came about. One school, which some refer to as the "Four Foundations", (see Domestication and surviving wild species, below), suggests that the modern horse evolved from multiple types of early wild pony and horse prototypes, each adapted to a given habitat, and the differences between these types account for some of the differences in type of the modern breeds. A second school - the "Single Foundation" - holds only one type of wild horse underwent domestication, and it diverged in form after domestication through human selective breeding (or in the case of feral horses and landraces, through ecological pressures). DNA evidence appears to show that domesticated horses evolved from multiple wild populations, instead of only one.

In either case, modern horse breeds developed in response to the need for "form to function"; that is, the necessity to develop certain physical characteristics necessary to perform a certain type of work. Thus, light, refined horses such as the Arabian horse or the Akhal-Teke developed in dry climates to be fast and with great endurance over long distances, while heavy draft horse such as the Belgian developed out of a need to pull plows and perform other farm work. Ponies of all breeds developed out of a dual need to create mounts suitable for children as well as for work in small places like mine shafts or in areas where there was insufficient forage to support larger draft animals. In between these extremes, horses were bred to be particularly suitable for tasks that included pulling carriages, carrying heavily-armored knights, jumping, racing, herding other animals, and packing supplies.