This is not a horse trailer. Road transport of the automobile began in the early 1900’s. however using horses as transportation was still more commonly used rather than an automobile. There were a few “home built”  horse trailers, Vehicles were not yet fully capable of handling  the extra weight of a trailer; Let alone with the added weight of a horse. Therefor transportation of horses by an automobile and trailer were kept short and was most likely only used to transport injured horses. Since It was easier and faster to just ride the horse to were you needed it. In fact horse trailers were not being manufactured commercially until 1958.