Con-artists have entered into written leases and then disappeared with the horse. People are more apt to follow the rules if there is a contract. Law enforcement considers breach of contract a civil matter. Expect to go into court yourself or be prepared to hire an attorney if the person leasing your horse breaks the contract. It is more difficult to enforce a civil contract across state lines.
Experienced horsemen know how much it costs to maintain that horse.
They may have a high maintainance horse on their farm, but that is because that horse is special to them . Your horse is not. People will tell a horse owner whatever the owner wants to hear so that the horse owner will sell or give the horse to them. Vet, farrier, and the dentist. Consider tattooing, branding, or microchiping the horse for later identification.