The only real ways I see OP getting out of this is if it was something along the lines of seller representing the horse as quiet and with good manners but when OP arrives, the horse turns out to be green broke and wild as they come. Or, seller represented the horse as being a 2nd level dressage competitor when in reality, horse barely makes it through training level. Drastic difference. A simple "not the personality I wanted" more than likely won't fly as a legitimate reason to back out of a sale.