What to try?

Horse #2, Bay, has some scruffy stuff on his neck. He seems to be prone to these weird conditions. Last year it was hives. It feels like a bump under the hair. If you scratch at it, it comes off like a scab, along with his hair and leaves a bald patch of skin underneath. We washed his neck and mane on Sunday and applied a fungicide. Don’t know if it’s a fungus or not. It doesn’t seem to be causing him any pain. I thought maybe it was rain rot, but none of the other horses seem to have it. We went to try and buy him a neck cover just in case, but that’s an exercise in futility. Looks like we’ll have to try and find one on the internet unless we want to buy an entire new blanket too.

Has anyone else had this problem? Any suggestions on what it could be and steps to take before the vet is called in?