As has been pointed out by a few, you can enforce the breach of contract, but it wont get the horse back, even if they know where the horse is. All they will cop is a fine for breaching contract and if your lucky, your court costs.
Also guess what happens to people that dont pay out on court orders? you have to keep taking them back to court to get a shiny new order. Maybe eventually they go to jail for contempt, but highly unlikely in this case. You can try collection agents and bayliffs, but how far are you going to go to get the 500$ fine they got, you aint getting the horse back either way.
The only way really to stack the deck is to lease it and retain ownership, and even then horses dissappear. This can fall into criminal law (rather than civil - contract breaches) and the perpetrator can be charged with theft. Ive not had a horse stolen, but ive been a victim of theft, and I got a nice letter in the mail to tell me the person involved was convicted. I didnt get any of my money back, the court didnt even order any restitution for the victims. Horses come under property in law, so it would be a pretty exceptional set of circumstances for you to get the horse back this way too, like they still had it with the rest of the stolen property when they were raided.
I dont know why people think they can have their cake and eat it too with horses, ive never seen anyone sell a car and request its returned if unwanted, or sell a house with a buy back clause. Horse people need to understand horses are property under the law. If you sell it, its sold, thats a contract under law, and thats where your control ends.