Feathering, or feather, is a term used to describe the long hair on the lower legs and fetlock

Fetlock

Fetlock is the common name for the metacarpophalangeal and metatarsophalangeal joints of horses, large animals, and sometimes dogs. It is formed by the junction of the third metacarpal or metatarsal bones proximad and the proximal phalanx distad...

s of some breeds of horse

Horse

The horse is one of two extant subspecies of Equus ferus, or the wild horse. It is a single-hooved mammal belonging to the taxonomic 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...

 and pony

Pony

A pony is a small horse . Depending on context, a pony may be a horse that is under an approximate or exact height at the withers, or a small horse with a specific conformation and temperament. There are many different breeds...

. On some horses, especially draft

Draft horse

A draft horse , draught horse or dray horse , less often called a work horse or heavy horse, is a large horse bred for hard, heavy tasks such as ploughing and farm labour...

 breeds, the hair can almost cover the hooves

Horse hoof

A horse hoof is a structure surrounding the distal phalanx of the 3rd digit of each of the four limbs of Equus species, which is covered by complex soft tissue and keratinised structures...

. While nearly all horses will grow longer hair on the lower legs and back of the fetlocks at times, particularly in the winter, "Feather" refers to the particularly long, luxuriant growth that is characteristic of certain breeds.