It’s a chestnut from Lightning’s front leg!
Donkeys and Zebras have chestnuts only on the inside of their front legs, while most breeds of horses have them on both front and rear legs. Scientists believe chestnuts are vestigial remnants of toes, wrist pads or scent glands that equine have evolved without. They are areas of thickened skin that painlessly shed a few layers over time. It’s often thought that the skin prints visible on chestnuts are as unique to individual equine as fingerprints are to humans.