* Early forms: Horse-drawn carts for transporting goods existed long before the modern horse trailer. These carts were likely used to transport horses as well, but not in a way that would be considered a dedicated trailer.
* 19th century: The first recognizable horse trailers likely appeared in the late 19th century. These were simple, often wooden structures with a basic open design, primarily used for transporting horses to and from events.
* 20th century: The 20th century saw significant advancements in horse trailer design. Metal construction became more common, and features like ramps, stalls, and ventilation systems were incorporated. The development of the automobile also led to the introduction of horse trailers that could be towed by cars.
* Modern era: Today's horse trailers come in various sizes and styles, offering advanced features like air conditioning, automatic leveling systems, and even custom interiors.
Therefore, while we can't pinpoint a specific date, it's safe to say that horse trailers emerged and evolved throughout the late 19th and 20th centuries, with continuous refinements in design and functionality leading to the modern horse trailers we see today.