Chestnut (horse anatomy)

The chestnut, also known as a night eye,^[1] is a callosity on the body of a horse or other equine, 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 believed to be a vestigial toe, and along with the ergot form the three toes of some other extinct Equidae.^[2][3] Darren Naish dissents from this belief, noting that the chestnut is "not associated with the metacarpus or metatarsus, the only places where digits occur."^[4]

Chestnuts vary in size and shape and are sometimes compared to the fingerprints in humans.^[1] For purposes of identification some breed registries require photographs of them among other individual characteristics.^[1] However, because chestnuts grow over time and horse groomers often peel or trim off the outer layers for neatness, their appearance is subject to change.