Of course make sure the trailer is secure and that your horse won't be jostled all over the place as you drive. Some things that are required to take with you are a vet certificate, proof of vaccinations, proof of ownership, a first aid kit that includes emergency numbers. Check for the availability of equine vets along your travel route as a precaution. Each place you travel through may have other regulations. Map out your route and check beforehand about their regulations.