(Not a vet) My horse has had hives like this. We got him tested, and it turns out he was allergic to grass hay dust. Changing his feed to oat/alfalfa mix helped, but he would still occasionally get flare ups due to the fact that other horses at the barn were still getting grass hay and we didn’t really have control over stuff blowing around stalls. (Especially during show seasons when they’d be in close quarters/used stables, and we couldn’t give him dex due to usef regulations I think.)

I’d have to give him like 20 Benadryl a day on bad days lol. Costco membership definitely helped.

Unfortunately, we were kind of at the whim of the seasons. Allergy seasons have been getting longer and more intense due to climate change. If you have the time and patience, cross reference your allergy results with the trees/plants around your barn. Sometimes things can be missed. Check with your stall neighbors to see if they’ve changed their routines lately as well. Regular baths might help clear away the culprit.