I have a Rocky Mountain mare, about 5 years old. I went out to saddle her the other day and when I went to groom her before putting on the saddle, I saw that her hair was coming out in huge clumps. I could rub my hand on her and get big hand-fulls of hair. Under it was shorter, darker hair, so it was not exposing the skin, but it sure seem very odd to be losing hair like that at this time of the year. She is on her feed like normal and doesn't show any other signs of distress, so I'm at a loss. Any ideas?
Not sure from your description if what you are seeing is different than normal seasonal shedding.
Horses (and dogs and such) shed two times per year. The most noticeable is the when they shed in the spring to get rid of their thick and long winter coat and get their summer coat.
They shed again when the days start getting shorter to grow in their winter coat.