The coin isn't showing well in the photo. If it's a nickle, it's probably off a bridle or head stall for a saddle horse. After the horse has been ridden for awhile, I don't know what it is, but their cheek itches or they want the bit out of their mouth or what ever, if the critter is tied up, perhaps to a tree, and you go to answer natures call, or get a drink from the spring, or stalk a nice buck you just spotted, the horse will just rub the daylights out of the side of it's head against the tree, sometimes rubbing the bridle off or even breaking the leather strap. Just my SWAG from experience.