I agree with the "no monkey business" comment. We can all guess whether or not a contract provision is enforceable. However, the existence of a contract provision that you knew about and agreed to almost certainly points to a legal battle with a judge deciding the outcome.
Even if you end up being allowed to move the horse, there will likely be a financial responsibility.
I may "sell" you a horse for $1.00, but intend on making up for that through boarding fees. In effect, the boarding stipulation becomes a payment plan.
If you want the ability to move the horse, don't have anything in the sale contract that infringes that ability. A good compromise may be to have a specified time for boarding your horse there - 2 years, 5 years, etc. Along with that you should also have a method of determining how much money is owed if you decide to move your horse early. Sort of like the "early termination fee" in a cell phone contract. This way, both you and the seller know exactly where you stand and there is no need for a court to decide who is right and how much is owed.