The least expensive type of trailer is the stock trailer, a trailer designed for cattle that is enclosed on the bottom, but has slits at roughly the eye level of the animals to allow ventilation. Trailers designed specifically for horses are more elaborate. Because horses are usually hauled for the purpose of competition or work, where they must arrive in peak physical condition, horse trailers are designed for the comfort and safety of the animals. They usually have adjustable vents and windows as well as suspension designed to provide a smooth ride and less stress on the animals.