My gelding did the same. What I had to do with him was practice A LOT. I don't own a trailer myself but anytime someone had a trailer hitched up, they knew of my dilemma and would let me "borrow" the opportunity. What I ended up doing was using a friends big Featherlite trailer, which had a feeding area. I opened up all the doors and windows to make it less dark and more inviting. When I went out to the paddock to get him, at feeding time, I didn't take him to his stall. Instead, I took him to the trailer and put his grain in the feeding area of the trailer. At no point did I tug on him or use any force to get him into the trailer. I decided he was going to reason with himself that going into the trailer was a BIG reward...dinner. The first time took more than an hour. It was hot, during the summer and I was standing inside the trailer, shaking his grain bucket...I was HOT. But I was patient and never got upset with him. He finally came inside the trailer and there were a few times he backed out on his own. But then he came back inside to finish his dinner. I did this everyday for a while and he learned that the trailer equalled pleasant things...food, neck scratches, praise, treats. I just had to be patient. But I believe that if you start punishing your horse and using whips, and tugging, pulling, etc. they will learn to associate the trailer with unpleasant actions and will hate it all that much more. Sadly, though the ending wasn't good for my gelding because he had a trailering accident a few years later and is back to thinking the trailer is a big monster that will get him. The only way I can haul him right now is in a big open stock trailer. I really need to buy a trailer and get him used to loading and traveling again.