The magic shop has magic tack for sale.
The shape of the magic tack is 1000 and it is unbreakable, so once worn by a horse it will last a lifetime.
When the magic tack is assigned to a horse you have the option of selecting between standard tack or specialized one. Make that choice carefully as it is permanent.
Note that it might take several minutes for the tack to become visible on the horse.
Also, once you give an item to a horse it can no longer be removed.