Butt...rash?

So when I was grooming my quarter horse today, I noticed he has this fungus-looking possible rash thing on the left side of his butt on the skin right next to his tail.
I wish I would've gotten a picture of it, but it almost looks like really mild mud fever, with little tiny brown bumps, sort of like an infection. It doesn't seem to hurt him when I touched it, and he doesn't ever itch it. Has anyone else ever had this issue?
I'll post a picture next time I get out to the barn.

Sounds like rain scald. Give him a good scrub with medicated shampoo and keep the area dry