If you haven't been involved with both activities, you may not know the difference between motorcycle chaps and horseback riding chaps, however there are a few main differences. They are activity specific and should really not be interchanged. Here are a few qualities about each.
Motorcycle chaps will almost always come in leather.Horseback riding chaps though come in many different materials and are often made to look professional and dressed up or to match an outfit when showing a horse or riding in a competition. While horseback riding chaps do come in leather many times, they also come in vinyl. More often than not, horseback riding chaps have a fringe on them and motorcycle chaps are seeing less of a fringe now than ever before.
Also, horse back riding chaps have been designed for many different purposes. Depending on where you are riding you will have different chaps for the terrain that you are on as well as the climate. They are also geared toward which type of activity you are doing whether it be straight forward riding, shows or even tricks.
While horseback riders have a history of wearing chaps to protect themselves. This tradition dates all the way about to 1870 when they rode horseback and showed off their horses in competitions. Motorcycle riders on the other hand, haven't been wearing chaps quite as long. The word 'chap' actually came from a horseback riding culture and then as they have expanded and developed more for other activities such as motorcycling riding, they have used the same word.
While motorcycle chaps definitely have a purpose and are worn often by serious bikers, they are not really necessary for the activity. If you are horse back riding, you will almost always wear chaps. Not only does a pair of chaps become a familiar feeling for the rider, but it is also a feeling that keeps the horse consistent as well. Riders need to maintain contact with the saddle while riding a horse so that they can stay in control and keep the horse in line. A biker on the other hand does not need to have a pair of chaps on to control a bike.
While both motorcycle and horseback riding chaps do provide the rider with protection of their legs, the type of protection is much different. A motorcycle rider is being protected by the heat of the bike as well as any debris or objects that may hit when riding in the street. A horseback rider is being protected from the horse and burns that are similar to rug burns on their legs. When you are riding a horse your legs beat on the side and can end up with bruising, burns or irritation. The chaps help to keep all of these at a minimum.
It is clear that both have similar purposes and that they are used for protection of the rider, however they have distinct differences as well.