I feed ACV. I feed 1/2 cup in their feed, 1 cup during fly season. It seems to help with the flies some, keeps the "boys" a bit cleaner, prevents rain rot, and helps mask powdered supplements or medications. The only negative would be if you have an ulcer prone horse. ACV is bad for ulcers.
I have not noticed any difference with my arthritic horses, but I feed the cheap ACV from wal-mart. Friends that use it for arthritis use the organic unfiltered ACV with the "stuff" at the bottom. Shake it well and feed 1/4-1/2 cup daily.