I'm going with a form of rain rot and ringworm, not something scary and exotic. I'd curry it out, get as much dead skin and hair off as I could, get the area clean, and scrub it with a strong percentage iodine and possibly peroxide at the same time, let it set a few minutes, then rinse out, pat dry, keep clean. It should heal up just fine.
Alternatively - I've seen black cattle flies do this to my own horses in the heat of summer. Usually just scrubbing it clean, shampooing well, and letting it dry before a good fly spraying takes care of it.
"We are here to laugh at the odds and live our lives so well that death will tremble to take us."