Horse trailers exist so that people can get their horses from one place to another without needing to ride them.  Traveling at highway speeds is a much faster way to get a horse from point a to point b.  Horse trailers can be steel or aluminum or a combination of both.  Some horse trailers have RV type living space and can hold many horses.  The largest horse trailer I have seen can pull 12 horses.  Straight, slant and reverse slant are all different load types.