A hard callosity found on the inside of a zebra’s leg called a chestnut.
When viewing zebra while out on safari, one can determine through a simple trick whether anyone on your safari vehicle does or has done horse-riding before by pointing out a dark circular mass growing mid-way up the front leg of a zebra and asking if anyone knows what this could be.
If there is an immediate answer, either you have a horse rider present or an equine vet! These odd looking callouses (for that is what they are) are found on most equine species and are called Chestnuts or even Night Eyes. They are thought to be either vestigial toes from a time when horses had more than one toe or otherwise vestigial scent glands much like those found on the hind legs of impala and other antelope species.
These calluses serve no purpose to the animal in question and in some cases, can be used in identification of a particular animal seeing as these chestnuts have been described as akin to fingerprints in their individual appearances. This is sometimes not as accurate as you would want it to be as these calluses are constantly growing, exfoliating and changing as the animal grows.
Blog by Sheldon Hooper (Bush Lodge Ranger)