Horses get chestnuts because they’re carrying a physical remnant of their evolutionary past. These rough, callous-like growths on the inner legs are vestigial structures, leftover from a time millions of years ago when the ancestors of modern horses had multiple toes instead of a single hoof. They serve no active function today, but every horse grows them naturally as part of its skin.
What Chestnuts Actually Are
Chestnuts are patches of thickened, keratinized skin found on the inside of a horse’s legs. On the front legs, they sit just above the knee. On the hind legs, they appear just below the hock. They’re made of keratin, the same tough protein in hooves and human fingernails, and they feel like a dry, flaky callous. They tend to be more pronounced on the front legs. On the hind legs, they’re smaller, and in some breeds they’re missing entirely.
Under a microscope, chestnut tissue looks nothing like regular skin. Normal leg skin has four to six cell layers in the outer epidermis, along with hair follicles, sweat glands, and oil glands. Chestnut tissue has over 30 cell layers of compacted keratin, with no hair follicles or glands at all. It’s essentially a dense pad of dead skin cells that keeps building up over time, which is why chestnuts can look rough and overgrown if left untended.
The Evolutionary Explanation
Around 55 million years ago, the earliest known horse ancestor, a small forest-dwelling animal called Eohippus (the “dawn horse”), walked on multiple toes. It had four toes on its front feet and three on its back feet, suited for navigating soft, forested ground. Over tens of millions of years, horses evolved to live on open grasslands where speed mattered more than footing on soft soil. Their bodies adapted accordingly: the toes gradually fused and reduced until only one remained, forming the single powerful hoof horses run on today.
The chestnuts are what’s left of that process. The leading theory is that they correspond to the wrist and ankle pads that multi-toed animals used, similar to the paw pads you see on dogs and cats. As the extra toes shrank away, the pads that once covered them didn’t vanish completely. Instead, they persisted as these small keratinized patches. Horses also have a related structure called an ergot, a smaller nub hidden in the hair at the back of the fetlock joint, which is thought to be the remnant of a different toe pad or dewclaw.
The Scent Gland Theory
There’s a second, less widely accepted theory: that chestnuts are the remnants of a scent gland rather than a toe pad. Some researchers have suggested that Eohippus or its descendants may have used glands in this location for chemical communication, and that the chestnuts are what’s left after that function was lost. People who handle chestnuts often notice they have a distinct, smoky, slightly sweet smell. Whether that scent is a genuine leftover of glandular function or simply the natural odor of concentrated keratin isn’t settled. Most anatomists favor the toe pad explanation, but the scent gland idea hasn’t been fully ruled out.
Why They Vary Between Horses
Chestnuts differ noticeably from one horse to the next. Some horses grow large, layered chestnuts that peel away in thick flakes. Others have small, flat ones that barely stand out from the surrounding skin. This variation is partly individual and partly breed-related. Most domestic horse breeds and Przewalski’s horses (the last truly wild horse species) have chestnuts on all four legs. However, a few breeds lack them on the hind legs entirely. Front leg chestnuts are almost always larger and more prominent than hind leg ones, even when both are present.
Because each horse’s chestnuts have a unique size, shape, and surface pattern, they’ve sometimes been compared to human fingerprints. Some breed registries have historically used chestnut patterns as an identification tool alongside other markings.
Chestnuts vs. Ergots
Chestnuts and ergots are often confused, but they sit in different locations and likely trace back to different parts of the ancestral foot. Chestnuts appear on the inner leg near the knee (front) or hock (hind). Ergots are found on the underside of the fetlock joint, at the back of the ankle, often buried under a tuft of hair. Ergots are generally much smaller in diameter. Both are made of keratin and both are evolutionary leftovers, but chestnuts correspond to wrist and ankle pads while ergots are thought to be remnants of a toe or dewclaw structure.
Grooming and Care
Chestnuts grow continuously, like a fingernail. They don’t cause pain or health problems, but they can become large and rough-looking if ignored. Many horse owners peel or trim them for cosmetic reasons, especially before shows. The easiest approach is to soften them first with a bit of moisturizer or after a bath, then peel the outer layers away by hand. They pull off in flaky sheets once softened. Trimming with a farrier’s knife is another option, though most owners find peeling sufficient. There’s no nerve supply or blood flow in the outer layers, so removing excess growth doesn’t hurt the horse. You’re only taking off dead keratin, the same as trimming a fingernail.
How fast chestnuts grow and how thick they get varies. Some horses need their chestnuts tidied every few weeks, while others barely accumulate any buildup at all. There’s no medical reason to remove them unless they’ve grown large enough to catch on something or become irritated by tack.