I have several friends who dilute this with another fly spray, or vinegar, and the awfulness of it diminishes enough they aren’t clutching their chest, but still works just as well
There’s a giant difference between things that get liberally sprayed on the “ground”, and worked into the soil by rain, and potentially then into the watershed, and something you spray on a horse. I wouldn’t at all be concerned about the tiny amount of chemicals in fly sprays that randomly might drip to the ground after a rain. And it’s amazing how much the soil breaks down some of the chemicals as they percolate down through the many feet to the watershed, if they even make it that far.
Maybe short-term, like for a ride. If you add some mineral oil to the mix and shake before using, you can extend its effectiveness for a bit longer. Water-based evaporates fast. Essential oils evaporate pretty fast, and you need some sort of oil to get them mixed into water to any significant degree
Flick’s is a natural spray I have found most effective for a ride
Otherwise, It’s things like UltraShield black, Repel-X (same dilution ratios as McGurk does for the same reasons), Pyranha (or Dumor Platinum fly spray (silver bottle, not white) from TSC which is the same thing but cheaper), are the things that seem to work the best, for the longest. Tri-Tec is another.