Crossing state lines with a horse doesn't have to be as complicated as it might sounds. To legally cross state lines with a horse or multiple horses in your horse trailer, there are two basic legal documents you need to acquire and keep with you while you are crossing state lines.
1. Never cross state lines with a horse without proof of a current negative coggins test. Each year during your horses vaccination your vet should draw blood to run a Coggins Test for Equine Infectious Anemia. The paper your vet provides proving a negative result should be carries with you every time you travel with or trailer your horse- even if you are not crossing state lines.
2. Always acquire a health certificate within 30 days prior to crossing state lines with your horse. A health certificate is simply a form with your starting point and destination filled in, with the signature of your vet confirming the horse described in the paperwork is free of any infectious diseases. A health certificate usually only costs $5-$10 if the horse has been recently examined by the vet.
Call your county extension agent or the Department of Agriculture for the state you will be traveling to to make sure additional paperwork is not required to haul a horse across state lines. Hauling horses across the state line into the state of Illinois, for example, requires a phone call to the Dept. of Agriculture to acquire a number. If you check local and state regulations and provide the important documents described above you should have no problems with your horse crossing state lines.