my horse was absolutely petrified of fly spray. i got some apples and held one out while I held the spray bottle and sprayed away from him into the air . I gave him an apple splice I did this for awhile and then sprayed lightly onto his legs. over time he's gotten better
My mare is also spooky about fly spray. What I personally do is start from the legs and go up. I spray it quite far so she gets used to it and little by little I spray it nearer. After I give her sone treats and lots of pats.