What are the dark horny structure found buried in the long caudal hairs of equine fetlocks are called?

The dark, horny structures found buried in the long caudal hairs of equine fetlocks are called "ergots." Ergots are small, keratinized growths located on the back of the fetlock, and they can vary in size and prominence among different horses. While they are generally considered a normal part of equine anatomy, their presence may be more pronounced in certain breeds or individual horses.