The layer of hair that grows thicker on a horse in winter for insulation is called its winter coat.
As daylight decreases in autumn, a horse generally develops longer, denser hair. The coat traps a layer of warm air next to the skin, reducing heat loss and helping the animal stay comfortable in cold weather. This seasonal change is driven mainly by photoperiod, or changing day length, although temperature and management also influence coat growth.
A horse’s winter coat is not a separate garment or a different type of fur; it is the horse’s natural seasonal hair coat. Horses can raise the hairs slightly to improve the trapped-air layer, while wet, matted, or flattened hair provides less insulation. Body fat, shelter, nutrition, and acclimatization also contribute to cold-weather protection.
A common mix-up is assuming every horse needs a blanket. Many healthy, acclimatized horses with an intact, dry winter coat can tolerate cold conditions, but clipped horses, elderly or underweight horses, and animals unused to a colder climate may need additional protection. In spring, the winter coat is shed and replaced by a shorter summer coat.