Pretty hard to tell. Looks like it was for a hind foot, and the remains of the groove make me think it was on the left hind foot. Trouble is, horse shoes just like it are being used today. I've worked horseback in very rocky/granite country, and we'd wear horse shoes in two in five to six weeks. Or, in soft soil, shoes can be reset on the horse's foot once or twice before replacing with new ones, and they don't get worn in two very often in soft soil, but it happens. Your best guess for the age of what you have, is what is the age of other relics you are finding in the same area. Hope this helps.