I use the Yard guard fogger all the time on mine. I would try a small area to make sure your horse doesn't react to it. At horse shows I find a mixture of yard guard and regular fly spray works the best as there are such super bugs out there that are immune to it all it seems,l ol
I wouldn't use it because the horse might ingest some of it when scratching himself with his teeth or when another horse grooms, etc. etc. Just doesn't seem like a good idea to me. I use vinegar, dish soap and water. Doesn't last as long though I'm sure.
We trail ride in some very thickly populated mosquito areas during the summer. Even with coating the horses in their normal fly spray beforehand, we always carry towels soaked in fly spray as well as a bottle of Off. Some of the horses are pretty sensitive to mosquitos and we have to wipe them down in the middle of the ride. When all else fails, we pull out the Off!
I've never used Off! Or any bug spray for humans on a horse before...I personally wouldn't risk it (for example, what if they ingest it?), so I just use fly spray/bug spray for horses. Like UltraSheild, it works really well. :)
I had a horse that was very alleric to insects. We had him a the vet school at Michigan State University for allergy testing and the supervising vet there told me to use Off or barn fogger. He said just avoid the Deep Woods Off as it has more DEET which isn't that great for horses. That was at one of the top vet schools in the country. I use regular spray at home for daily use but at the shows I use the more potent spray as the flies at fairgrounds seem desensitized to regular fly spray.