1) Have the tenants execute an agreement in which they agree to indemnify you (reimburse you) for any costs or judgments against you due to the horses, which includes any legal fees you might have to pay.2) Have the tenants give you a "horse" or "animal" deposit, which will represent a ready source of money to use in the event of any claim or cost.3) Make sure you have adequate insurance yourself, and require your tenants to maintain (and provide proof of) adequate liability insurance.