Diane Sasscer is an independent saddle-fitter who has been fitting saddles for 18 years. Her clients include Olympic team riders from the U.S. and other countries as well as local horse enthusiasts. She is based in Wellington, Florida. (Saddlefitbydiane.com).
The reasons for asymmetry are injury and lack of straightness through training. One has to determine the root of why the horse has developed this issue. Sometimes, it has less to do with the shoulder and more to do with a particular area across the horse’s back. For example, a large left shoulder is indicative of a weak right hind (or the opposite). It is normally a weak hind leg that is not stepping through. In this case, the muscle under the rider’s right seat bone does not develop due to the rider’s weight not being carried equally.