The horse skeleton averages 205 bones

Gestation lasts approximately 340 days, with an average range 320–370 days,[49] and usually results in one foal; twins are rare.[50] Horses are a precocial species, and foals are capable of standing and running within a short time following birth.[51] Foals are usually born in the spring. The estrous cycle of a mare occurs roughly every 19–22 days and occurs from early spring into autumn. Most mares enter an anestrus period during the winter and thus do not cycle in this period.[52] Foals are generally weaned from their mothers between four and six months of age.[53]
Horses, particularly colts, sometimes are physically capable of reproduction at about 18 months, but domesticated horses are rarely allowed to breed before the age of three, especially females.[54] Horses four years old are considered mature, although the skeleton normally continues to develop until the age of six; maturation also depends on the horse's size, breed, sex, and quality of care. Larger horses have larger bones; therefore, not only do the bones take longer to form bone tissue, but the epiphyseal plates are larger and take longer to convert from cartilage to bone. These plates convert after the other parts of the bones, and are crucial to development.[55]
Depending on maturity, breed, and work expected, horses are usually put under saddle and trained to be ridden between the ages of two and four.[56] Although Thoroughbred race horses are put on the track as young as the age of two in some countries,[57] horses specifically bred for sports such as dressage are generally not put under saddle until they are three or four years old, because their bones and muscles are not solidly developed.[58] For endurance riding competition, horses are not deemed mature enough to compete until they are a full 60 calendar months (five years) old.[11]
Anatomy
Main articles: Equine anatomy, Muscular system of the horse, Respiratory system of the horse, and Circulatory system of the horse
Skeletal system
Main article: Skeletal system of the horse
Diagram of a horse skeleton with major parts labeled.

The skeletal system of a modern horse
The horse skeleton averages 205 bones.[59] A significant difference between the horse skeleton and that of a human is the lack of a collarbone—the horse's forelimbs are attached to the spinal column by a powerful set of muscles, tendons, and ligaments that attach the shoulder blade to the torso. The horse's legs and hooves are also unique structures. Their leg bones are proportioned differently from those of a human. For example, the body part that is called a horse's "knee" is actually made up of the carpal bones that correspond to the human wrist. Similarly, the hock contains bones equivalent to those in the human ankle and heel. The lower leg bones of a horse correspond to the bones of the human hand or foot, and the fetlock (incorrectly called the "ankle") is actually the proximal sesamoid bones between the cannon bones (a single equivalent to the human metacarpal or metatarsal bones) and the proximal phalanges, located where one finds the "knuckles" of a human. A horse also has no muscles in its legs below the knees and hocks, only skin, hair, bone, tendons, ligaments, cartilage, and the assorted specialized tissues that make up the hoof

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