I’ve been overtaken by a horsetailer doing a bit over 100mph. I do believe it was a Porsche suv doing the pulling tho. This was in Denmark. Back when I was young and reckless my old Peugeot sedan was pretty stable doing around that speed with a flatbed dual axle trailer.

The horse trailer probably didn't contain a horse at the time. They're very useful for hauling all sorts of things, most of which are lighter than horses.