I had the same problem last year, out of nowhere my mare starting loosing her beautiful and long tail hair right from the root. She wasnt rubbing either. We figured it was the no-see-um bugs, evidently they come out when it starts getting moist out? I used a medicated shampoo, saw a little result, but not much. It ended up going away once the weather dried up a bit. Hoping it doesnt come back this winter.