You have to remember that horses are prey animals, and by nature are fearful of confined, dark places. They don't know if there's a way out, if a predator is in there or if they may get stuck. This seems silly when it comes to dealing with domesticated horses, but they definitely maintain some of those natural fears and instincts no matter how "broke" your horse is.
In order to get horses over that fear, you have to make the trailer inviting and less scary. Having excellent ground respect will help, for starters. It also helps to use DISCIPLINE, not PUNISHMENT. If your horse gets up to the trailer, puts a foot on the ramp then picks up his head and backs off, do NOT snap the rope, jerk the halter, shout or make him more nervous. If you do this, he learns that he gets PUNISHED for even trying, and it will make him more hesitant to get on. If you let him take a step back calmly, praising his step up first, then apply even pressure on the rope until he comes forward - RELEASE as soon as he comes forward. This way he learns he is rewarded for trying by the removal of pressure.
Discipline is only applied if the horse does something DANGEROUS, like leaping forward into the trailer - especially if you are in it! - rearing, kicking at rump ropes or bolting. As I said before - don't snap ropes, jerk halters, smack the horse with a rope or scare him. Apply discipline by MOVING HIS FEET. If he gets pushy, tries to run you over or is in danger of harming you or a helper, back him out of the trailer and do some ground work to get his brain THINKING rather than REACTING - make him yield his hindquarters, lunge him calmly, back up, send him off, etc. Then approach the trailer and try again.
If food motivates your horse – use it! Sometimes horse associate trailers with shows, vets or other "unpleasent" things. It was suggested in this thread to put the trailer in a confined paddock and training your horse to eat his grain inside. This is an excellent approach for horses that are very food motivated and great for others. He'll eventually associate going in with eating dinner and not about where he's going.
If having a “trailer buddy” for you or your horse makes everyone calmer, use it! Some horses are very "herd minded" and while they don't mind being alone, loading into a trailer alone is a daunting task. Just having a "buddy" in the vicinit that's an old pro at loading that can be an example (load the pro in, then try the other horse and the same with backing out). Sometimes learning from others is a good way to show him that it's not so scary. And having a helper for YOURSELF is sometimes a good idea - they can watch out for safety reasons, help load, keep you calm and provde another set of eyes to refine your technique.
There are lots of different strategies to getting your horse on a trailer and over their fear of loading. Try out a few and see what works best, and if you end up taking a few techniques and combining them, then that works too! Good luck!
Last edited by ReiningGirl; 10-05-2011 at 02:54 PM.