I wonder if they could be some sort of allergic reaction to a horse fly bite or something like that? I really don't know. My friends horse was allergic to flies (the vet thought) and would break out with the huge weeping oozing wheels and hives. They would resolve in a few days. I don't remember them being that open though. Can you find anything that he could be scratching himself on that would or could leave that sort of mark?
Scorpions are all around where I live, in southern California. I live up in the mountains.