One poster mentioned lopsidedness in the horse, which is very common with
horses with one underdeveloped shoulder. With such a horse it's a good idea
to check to see if your horse has a club foot on the side of the
underdevelopment. What classically happens is that the shoulder is
underdeveloped and a dip occurs behind the wither into which the rider drops.
You can't do anything but ride lopsided.
To check it out, get on a stool behind your horse, get your horse to stand
square and look down his topline. You can see how even the shoulders are and
if there are irregularities in the topline and the withers.
To fix it, you can shim up the side with the dip and many times that works.
It didn't work with my mare so I just chose to ride that way and go to the
chiropractor weekly to have my legs brought back tothe same length. I noticed
a huge difference when I bought another horse that was balanced in both
shoulders, and my chiropractic visits diminished while riding him. Of course
chiropractic visits vary according to how many times you fall off, too. ;-)