Well technicaly never use "will" in a contract. Legally that mean she has all the time in the world.
Second issue is though she cant register because of the dame, Before you can sue you have to try to solve the problem. Send her a certified letter saying sense the horse was falsly represented and you are unable to fulfill the contract you will be returnign the horse and expect a full refund at that time. If she hesitates in the slightest you then need to file suit in small claims court. Sounds like a pretty simple case if you have things in writing and might be worth the $500 bucks for a lawyer to handle. Call a round for a couple free consults and get some quotes.
Lesson learned, never pay in full until you have every thing.