what is an ergot on a horse

An ergot is a small callosity or hard, horny growth on the underside of the fetlock of a horse or other equine. It is also known as Calcar metacarpeum and Calcar metatarseum. Some equines have ergots on all four fetlocks, while others have few or no detectable ergots. In horses, the ergot varies from very small to the size of a pea or bean, with larger ergots occurring in horses with "feather" – long hairs on the lower legs. In some other equines, the ergot can be as much as 3.8 cm (1.5 in) in diameter. The ergot is thought to be a vestige of some part of the ancestral foot of the multi-toed Equidae, corresponding to the sole pad of other extant members of Perissodactyla, such as the tapir and rhinoceros. Unlike the chestnut, which in the same individual may be large on the forelegs and smaller or even absent on the hindlegs, the ergot is of roughly equal size on all four legs.