Air conditioning units for horse trailers can be installed in the living quarters area or the dressing room area of your horse trailer. This is a great way to make your horse trailer cooler and more comfortable and itâ€s a common addition in many custom made horse trailers. If you want to keep the horse area of your horse trailer cooler as well, you can add horse trailer stall fans for extra ventilation. This is an excellent and affordable option, and is highly recommended if you plan to trailer a lot on hot summer days.