I can’t say my Palomino QH has had hives as often as you two, but I’ll say when I come across it, usually it’s after a hot day with tons of bugs/flys/ and all those nasty flying things.
We do spray and sheet our horses, but his neck is always exposed.
I usually rinse him with cold water, and walk him to cool him down since he is usually sweated. After he is cooled off I dry him with a towel and apply Vetryicn (spelling?) for topical skin irritations.
Though I don’t have much about solving this, please consider this:
Maybe the horse had eaten a bad plant in pasture
or, Rolled on a plant with oils that they reacted to.
or, Bug bites? or stress?
also, I wish the benedryl wash was still in manufacturing, but I know horses can be given Benedryl Allergy Pills. I don’t know the proper dosage. Ask your vet and see. I know we administer the pills to our show dogs who have bad allergies. It is 2 pills morning and night for a normal sized golden, 70 to 80 lbs?
All of these ideas came from people who told me what causes them for their horses. I believe my QH had reacted from bugs/flys both times since those were the worst seasons the horse flys were around. Good luck!