The most common causes of saddle sores on horses are by and large incorrectly fitting saddles. Similar to walking a mile or two in a pair of shoes the wrong size, being ridden in an ill fitting saddle can be extremely painful for a horse. If you suspect this may be the case (ill fitting saddles are also frequently accompanied by behavior problems) you should have your saddle fit checked by a trainer, vet, or (preferably) a professional saddle-fitter (who can be located through most tack stores) Saddles sores caused by poor saddle fit usually show up around the withers, shoulder, or back under the cantle.
Sometimes saddle sores are caused by a dirty saddle pad (or a clean saddle pad placed on a dirty horse) or a saddle cinched up with the hair not laying flat underneath. Both of these problems can be big enough irritants to cause open sores or wounds within the space of a short ride.
Finally, saddle sores can be caused by girths and cinches being too tight, too loose, too rough, too dirty, or skin being bunched underneath them- also galled girth galls.
Care for horse saddle sores is basically the same regardless of the cause. Treatment consists of a daily application of some type of wound care (or examination by a vet if severe enough) and absolutely no riding until the saddle sore has healed completely. After the skin has healed, riding can resume- with the initial cause of the sore removed. You may want to apply MTG to help the hair grow back, but many times white hairs will grow permanently where the sores were. To help prevent future sores from a girth, you can apply a salt-water solution to the healed skin once a day to help toughen the horse's skin.