My paint horse does this late every spring. He has a beautiful long mane, and he rubs a big chunk out of it. Then he stops doing it. It's grown out by the next year, when he promptly does it again. The vet was stumped, but I think I finally figured it out. I think he has sweet itch, allergic to gnats. What I found that REALLY helped is a product called M-T-G. Seems to stop the itching, and promotes the hair growing back. It's greasy and it smells like barbecue sauce lol, but it definitely seems to work.