I'm not sure where I picked this one up:
"Equal parts apple cider vinegar (do not use white vinegar) and Listerine mixed with baby oil (or Skin-so-Soft). Use 1/4 the amount of baby oil as vinegar and Listerine. It smells pretty badly, but it works for me and the horses I use it on, and I'd rather breath in that than the chemicals normally used in fly sprays. Before using it, give the bottle a good shake to mix the oil in with the other ingredients.
I was told that you can also use a sunscreen with this instead of baby oil (for sun sensitive horses), but I haven't. As always, before just going crazy and spraying this all over your horse, test it in an area to see how the skin reacts. I have heard of some horses break out in hives from this mixture, though I personally haven't had a problem, and I have used it on quite a few horses."