... but since then I have seen things for dogs collars that you clip onto the collar and when you turn it on, it is a light that blinks on and off. I did get some for my dogs and I can see them anyplace on our 1 acre fenced in yard. I wonder if that would for horses?
Reflective tape on a turnout-sheet or halter, or teaching them to come when caught.
My mare lives by herself in a 15 acre pasture at home. One night, we played spotlight in the pasture with some friends. It was humorous how my horse, associating coming when she heard/sniffed/saw me with treats and exercise, followed me around the entire night. It made hiding difficult, that's for sure!