What is a chestnut on a horse?

A "chestnut" on a horse refers to a dark brown or black horny growth found on the inside of a horse's leg, typically located just above the fetlock joint (though sometimes above the knee or hock). They're also sometimes called ergot (though ergot specifically refers to the similar growth found on the fetlock).

Here's what we know about chestnuts:

• Function: Their purpose is unknown, although several theories exist. Some believe they're vestigial remnants of what were once digits (toes) on the horse's ancestors. Others suggest they might have a sensory function.

• Appearance: They vary in size and shape from horse to horse, and can even differ between legs on the same horse. They're generally small and roundish, but can sometimes be elongated or irregular.

• Composition: They are composed of keratin, the same tough protein that makes up hooves and horns.

• Use in Identification: While not unique identifiers, chestnuts can be helpful in identifying a horse, particularly when combined with other markings. They are relatively consistent throughout the horse's life and don't change much in size or shape.

• Care: Generally, chestnuts require no special care. However, if they become excessively overgrown, cracked, or infected, veterinary attention might be needed.

In short, chestnuts are distinctive features of equine anatomy, whose exact function remains a mystery but are useful in identification and serve as an interesting evolutionary vestige.