A lower hip can also be caused by a horse not properly using one of his h ind legs for whatever reason, and the muscle on the hip of that side is underdeveloped. I have a mare with this issue. Her hocks and stiffles were stiff, more so on her left side. We had a couple of chiro treatments, did lots of stretching, and got her on a powerful joint supplement. We also work her over trot poles regularly. It's been 4 months and I can definitely see improvement in her gaits and her hip. It's not all the way level yet, but it's getting there.
My husband's gelding has a pronounced dropped hip. His was caused by a hip injury as a yearling. We did a lot of chiro and massage on him, trot poles and other exercises, and his hip is better, but it will never be level.