The horse (Equus caballus) is a odd-toed ungulate mammal, one of eight living species 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 4500 BC, with domestication of the horse believed to have been widespread by 3000 BC. By 2000 BC the use of domesticated horses had spread throughout the Eurasian continent. While most horses today are domesticated and live in close proximity to humans, there are still endangered populations of the Przewalski's horse, the only remaining true wild horse that has no domesticated ancestors, as well as more common feral horses, such as the American Mustang, which live in the wild but are descended from domesticated ancestors.

Horses are described with a large number of specific terms that describe everything from their anatomy to their life stages, colors, white markings, breeds, locomotion and behavior. They have a wide range of sizes, and are anatomically designed as a herbivore with the ability to use speed to escape predators. Horses 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 a unique trait: horses are able to sleep both standing up and laying 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 horses begin training under saddle or in harness between the ages of two and four, reaching 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 creating specially-developed breeds, particularly in Europe. There are over 300 breeds of horses in the world today, developed for many different uses. Some regions in the world are known for a specialty in development of a particular breed or set of breeds, such as harness horses in Russia or draft and pony breeds in Great Britain.

Horses and humans have extensive interactions today, not only in a wide variety of sports competitions and non-competitive recreational pursuits, but also working activities such as police, draft work, entertainment, assisted learning and therapy, and modern warfare. Many products are also made from horse meat, milk, hide, hair and bone, as well as pharmaceuticals made from the urine of pregnant mares.

Domesticated horses require specialized care to stay healthy, including proper provision of food, water and shelter, but also additional care from specialists such as veterinarians and horseshoers, or farriers. There is a wide variety of riding and driving techniques for managing horses, using many different styles of equipment and methods of control.