There is probably a term in your rental agreement with your tenants which will serve as the basis for you to demand that they remove the horses within a set period of time or else quit the premises. Does the zoning for your residential area allow horses? If you have a month to month lease then you can just give notice to the tenants that you are terminating the lease.

You need to speak to a "landlord tenant" attorney to evaluate your options, given the nature of your tenancy agreement with your tenants.

Good luck!

Brian Chase,

BISNAR|CHASE, Personal Injury Attorneys.
http://www.BestAttorney.com
http://www.California-Lawyer-Attorney.com
800-956-0123.