A horse I am riding has a skin condition and I have no idea what it is. It is in the base of her tail and on either side of the tail on the back of her. They are large, black flakes that look like skin and dirt? She is also sensitive to touch those areas. Could it be rain rot? On either side of the tail it is almost greasy looking with black specks. Does anyone know what it is? she came from a rescue a few months ago and had it there, too.
I'm gonna call that "Butt Dermatitis." Wash on and around the affected area with betadine or chlorhex scrub (leave on for TEN minutes BY THE CLOCK), and rinse. Do this for 14 days. That will fix 99% of Butt Dermatitis problems.
There is a Thread on this Forum on Onchocerca (Neck Thread Worms) that you may want to read. My horses had the same symptoms you are describing and my vet diagnosed it as Onchocerca. I treated him with Equimax as described in the Thread and it clears up fairly rapidly.
The thread is over 70 pages, but has a lot of very useful information.
If you'd like the specifics of what I did with my guy (since last August), just send me an email.
My vet said they can cause a breakout just about anywhere. My horses original symptoms were on his butt cheeks with the black scales as you described. He then progressed to where he broke out at the base of his mane (up under the mane where you couldn't see it).
I had never heard of it either, but from the thread on this site, it is very widespread. I think it is misdiagnosed a lot of times.