This horse was also a mare, 28 years old, I've found that cushings tends to develop in older horses, and in particular ponies. Iodine in the drinking water, copper sulphate mixed with sulpher in the feed, 1/2 teaspoon each in warm barley mash. The coat was back to normal in about 6 weeks. She also had swollen gums, which came back to normal, don't know if this was related to the cushings or not.