Tires in good condition? (look for dry rot, replace every 3-5 years regardless of mileage).
Check tire pressure (including spares and inside tire on dual wheels).
Hitch locked on the ball? Correct size ball?
Safety cables/chains connected?
Plug and secure electrical connection.
Connect emergency breakaway system.
Emergency battery charged?
Test trailer lighting (brakes, turn signals, running, perimeter).
Check/test brake controller.
Prior to loading horse(s), check trailer for hazards.
Leg wraps, head bumper on horse(s)?
Secure and lock all trailer doors.
Headlights on? For greater safety – get noticed.
Drive safely – allow greater braking distance, and travel at generally slower speeds.
Neva Kittrell Scheve is the author of The Complete Guide to Buying, Maintaining, and Servicing a Horse Trailer and Hawkins Guide: Horse Trailering on the Road. She and James Hamilton, DVM, co-authored Hawkins Guide: Equine Emergencies on the Road.