its seems to be the consensus the cause is the no-see-ums. My horse had the problem starting in the spring and it got worse over the summer. Its happened several years now.
I tried everything including the medicated shampoo. Hair just now started not coming out. I am going to start the combo of the listerine and bug spray earlier next year – maybe keep spraying on the listerine from now until then and see if I can prevent the next cycle. My horse’s tail is really thin now. She won’t have any tail left if this happens again next year.
Thanks for all of your replies. I really appreciated it.