Well, not having them even on a two horse sounds scary to me... but all horse trailers have to have them here, even the small older straight loads.
I wonder if it's flatland behavior where there aren't a lot of hills that people get away without them?
I don't know. The only reason mine doesn't is because it's so old and I bought it used. It has the cylinders for it, they're just not hooked up to anything and I never had it done. I don't know if they're required or not. I guess if I got stopped for something and they decided to be picky I'd find out the hard way, the same way with not having a tag on any of my trailers. We have some pretty steep hills here and my old 4WD Jeep does fine with it and 2 horses, but my 3-horse slant is just too much. One of these days I'm gonna get me a new truck! ;o)