Are you in a state with brand inspection laws? In colorado, whoever has the brand inspection owns the horse, regardless of bills of sale, registration papers, whatever. Brand Inspection trumps all.
If not, I believe that they are the legal owners - unless you lie in court about actually giving them the horse. I'm sorry to hear about your mare, though. Maybe bite the bullet and buy her back from them - in the end, it may be the only way to get her back.
There is no passion to be found playing small - in settling for a life that is less than the one you are capable of living. - Nelson Mandela