What does the horse want? Room and light, good ventilation, and safety in design.
Room and light: An average sized horse ( 15.1h - 16.3h) needs about 10 feet of usable length to be comfortable. A larger horse may even need more. A horse needs to be able to spread his legs for steadiness, but is also important that he be able to use his head and neck freely for balance. A light colored interior and lots of windows or slats will make the trailer much more inviting and less claustrophobic.
Good ventilation is important for the horse's respiratory health and to control the temperature and environment of the trailer. Hay dust and noxious gasses from manure and urine compromises the horse's respiratory system and predisposes him to diseases such a shipping fever. Roof vents will remove contaminated and/or hot air from the trailer. A light colored exterior, especially the roof, will make the trailer cooler in hot weather.
Safety in design: There should be nothing sticking out to harm the horse in anyway. Tie rings, and latches should fold flat against the wall. All center posts and dividers should quick release, but should be strong enough to not break apart until you can make the decision. (Exception - dividers should come up and out if a horse would get under it.) No sharp edges anywhere. All parts of the trailer should be strong enough to hold up to the largest, strongest horse who will be hauled in it.
Ramps should be low to the ground and not slippery. It should be possible to reach every horse individually in the event of an emergency. (This is a special problem with many slant load trailers.) Butt and chest bars should quick release.
Safety in design also includes road safety. All brakes and lights should be in perfect working order and the emergency breakaway brake battery should be charged. Safety chains on tag-along trailers must be crossed underneath, and ball hitch gooseneck hitches should also have safety chains or cables. Tires should be inflated to the recommended capacity, and rubber torsion suspension will not only reduce road shock for the horse, but will be an added safety feature if the event of a flat tire. It is most important that a tag-along trailer be hitched to a frame mounted Class III or Class IV hitch, and that the trailer be towed in a level position. Whether you are towing a gooseneck or a tag-along trailer, you must have a properly rated tow vehicle to insure your own safety.