There is nothing besides tincture of time that will work. That girl gave you a lesson in contract law and in loaning things to friends. You're lucky she's not ruined the horse's training. If I were you, I would ask her parents to pay you for the damages, if you can figure out some monetary damage, such as you expected a cash prize in a show for which you are now ineligible. If your horse was a champion show horse, with a career breeding potential, then the damage to you is the expected value of all the horse's offspring. Why? Because the show career is gone. A $50,000 horse is now a $1000 horse. If your horse was a top-dollar horse, then you should have the girl's parents find a mare of similar quality and buy it for you. Give them the horse that was ruined. Of course if your horse was not top quality, you could not do all that, but you can use the example to show them the kind of damage that was done. It's as if I loaned my car and someone ripped up the $3000 leather seats to put cheap beads onto them.