what is a horses chestnut

A horses chestnut is a callosity or hard, callus-like growth found on the inner side of the leg above the knee on the foreleg and, if present, below the hock on the hind leg. It is also known as a "night eye". Chestnuts are formed from small deposits of keratin that gather on the horse's leg, and they feel oily and scaly to the touch. Chestnuts vary in size and shape and are sometimes compared to fingerprints in humans. They are used for identification purposes by some breed registries, which require photographs of them among other individual characteristics. However, chestnuts grow over time and can be peeled or trimmed off by horse groomers for neatness, so their appearance is subject to change. The evolution of the horse involved a reduction in the number of toes to one, along with other changes to the ancestral equid foot, and the chestnut is thought to correspond to the wrist pad of dogs and cats. Alternatively, it may be a vestigial scent gland similar to those found in some deer and other animals.