Some people. I wouldn't want to send the horse back to him knowing he's not willing to get the horse vet care, or take care of him while he's healing from something, but at the same time, he does own the horse, you did not agree to any terms of taking the horse and paying for care etc. Definitely send the letter, along with a copy of any and all vet bills, and demand payment for the treatment, and the costs of "board" or feeding of the horse while in your care. A court if necessary would side with you, and you'll get your money somehow. Though hopefully it doesn't come down to having to go to court. I swear sometimes I wonder who people have managed to survive. I know someone who's always posting about her horse's problems, and asking for advice, but she always refuses to call the vet. It gets really old really fast. Sorry that the idiot just dumped the horse back in your lap.