Having towed both horse trailers and caravans , the horse trailers are notably more stable, they are usually twin axle and considerably shorter than many caravans, and also tend to have a lower centre of gravity
As a previous poster said there can be the best part of 2 tons of live animals inside,especially as often the trailer will have 2 horses loaded,
As a driver I have no way of knowing what state the horses are in , they may be nervous travellers, I also have no way of knowing the towing drivers experience, it may be the first time they have towed.
So I adopt a cautious attitude and hang back behind them, it can be frustrating , but so are many other things, including cyclists, road works and tractors, even caravans and motorhomes,but they all are part of modern life on the roads.
Better to be late in this world than early in the next.