When my little Arabian refused to trailer, the trainer came to my house and we stood on either side of him and each popped him steadily on the butt with a riding crop. Not hard enough to hurt, just hard enough to be annoying. If he moved toward the trailer, we would let up for a moment as a reward, if he backed away, we would increase the force. It took a long time, but eventually he got in, at which point, we let him out and did it again. Now I just send him in and he goes into the trailer first without hesitation. Your results may vary. And if you have a horse that kicks, I would solve that problem before trying this.
