When I’ve had occasion to haul horses loose, they have always ended up on an angle, and the younger one even turned to face backwards, on a slant.
The RCMP did a study with their big rigs, used heart rate monitors, and found that horses actually prefer to travel backwards given a choice. Wish I knew where that study was now.
I also think that a horse can lean on his butt when the trailer slows down, instead of lurching forward on his forehand. Seems would be less taxing.
My own custom made trailer is a slant and have no troubles putting in my large
17+ hand horses in there. It is overheight, too, giving the feeling of space. I think my horses travel better in it than the straight. In the past I’ve found that some horses cannot travel in the r.h.s. without scrambling, and this has not happened in the slant.
In the three, four, and longer horse trailers, there is no way to get the horses out in the case of an accident, but in a two horse, you can have an exit door put in (about a $1,000.00 option)
Overall, I vote for slant.