As has been said, never try to play lawyer. Even a real lawyer can miss something in a contract.
According to the terms you listed, there is nothing about condition or welfare of the horse. The only requirements is that the horse live outside, stay barefoot and be trimmed every four weeks.
They don't need to use you. They don't need to use a farrier. It doesn't even have to be a good trim. They simply have to buy a file and trim by themselves every four weeks.
Technically, they don't have to feed the horse or maintain its health. Of course, that would possibly get into the animal cruelty realm and involve various laws. The point it, it would not violate the terms of your contract, nor would it necessarily give you any rights to regain ownership of the horse.
If you are going down the "ribs showing" path there is something else to consider. Did you document the condition of the horse before you transferred ownership? What if the new owner claimed they received the horse in that, or worse condition? They could claim the _you_ gave them a starving horse that is now "on the road to recovery."
Sorry to sound so negative. Only want to point out that your contract does not give you much to stand on with the situation as described. If you want the horse back, a "honey" approach may work better than a "vinegar" approach (i.e. threats, SPCA, etc.).
(disclaimer - all opinion... I am not a lawyer)