I had a customer years ago whose warmblood gelding developed a winter-long case of hives. ALL options were explored (including veterinary diagnostics) and no resolution except warm weather was found. I do remember reading something that said cold can be a factor.
My horse had a major case of hives last week. Hauled him to the vet and she said this seemed to be a "banner year" for hives. She had seen SO many in just a few days. We had a ton of rain this year so Im thinking there must be a lot more pollen in the air than we usually have. (we mow the weeds/grass around his paddock so he cant get into things he shouldnt be eating)
This winter was very rainy and cold. We had snow a week ago. But some of the plants are trying to blossom in spite of the cold temps. So that would be my guess.
We gave my horse injectible benadryl (two doses) and the hives disapeared -so far have not returned.