Perhaps the most important accessories for a horse trailer are the ones that go on the horse. And the one the horse probably will appreciate the most is a head bumper. This is a "cap" for the horse that protects its head from bumps, either against the roof of the trailer when the driving gets rough or against the top of the door frame on the way in or out. Trailer pads can help keep the horse comfy while bumping up against the walls of the trailer. And so that nothing hits the horse during those same rough moments, a bungee cord is one of the best ways to tie down loose objects. An entry ramp for the horse might also be useful, but some experts believe that the horse would rather have some less shaky method of stepping up into the trailer, because the shifting motion of a ramp can be frightening.
Some of the best horse trailer accessories are also the most practical. A good emergency kit is essential. Accidents happen, and if one happens to your horse trailer, you'll be glad to have the supplies you need to take care of any injuries to your horse. (You might also want to keep a second kit, either in the trailer or the tow vehicle, for taking care of humans. And remember to carry a jack, should you need to fix a flat in a hurry.) A saddle rack will help you save room in your tow vehicle by stowing the saddle in the trailer. Feed bags will give your horse a chance to grab a meal while in transit; triangle feed bags are available with straps that attach to hooks inside the trailer. And because horses can't necessarily wait for a bathroom break, a muck rake will come in handy for cleaning up any unpleasant surprises your horse leaves behind.
Any and all of these accessories will be appreciated by your horse. However, the horse's owner might like a little external décor for the trailer. Several Internet vendors specialize in bumper stickers, magnets and decals for the trailer, with pictures of horses and clever horse-related sayings on them. Children riding in the tow vehicle would probably be entertained by any of the plastic models of horse trailers available from toy companies. And if you have some money left over after buying the essentials, you could even hire an artist to paint pastoral scenes on the trailer's side. The driver in the next lane will appreciate the view.
But the most important horse trailer accessory might just be a book about horse trailers. For instance, "The Complete Guide to Buying, Maintaining & Servicing a Horse Trailer" by Neva Kittrell Scheve and Thomas G. Scheve contains, according to Amazon.com, information on picking out a horse trailer and equipping it for the safety and comfort of the horse. And "Trailering Your Horse" by Cherry Hill and photographer Richard Klimesh provides a visual guide to getting your horse from place to place. Once you've finished learning about trailers, your horse might enjoy looking at the pictures.