Some kind of fungus, maybe?
Not sure what others would recommend, but if you're not going to have the vet come out, I'd hit it with some "first aid" type shampoo. ( http://www.finishlinehorse.com/produ...t-aid-shampoo/
for example. ) and then also some MTG. (Which stinks, but yields great results and is also said to be antifungal. It may actually be antifungal, but I haven't read the label in a few months. )
Good luck! Interested in what others have to say.
Also, if you're able to post pictures, please do so! It might help.