In the U.S., any slant trailer I've seen (whether it is normal slant or reverse slant) always has the horse's head on the driver's side of the trailer (left side). Likewise, the door to the trailer is either in the back of the trailer, or on the passenger side of the trailer (right side). In that regard, if you would need to unload your horse for some reason while pulled over on the side of the road, you want to be able to do that with your horse either behind the trailer or off on the passenger side.
Of course, if you live in a part of the world where you drive on the wrong side of the road (haha), then you may want the horse's head on the passenger side if you are going to have a side load. If the door is going to be in the back, then it may not matter as much.
∞•*˚ Βгįťţαňγ ˚*•∞
It is not enough to know how to ride; one must know how to fall.