Lights are not going to deter coyotes from wandering around by the barn at night. They don't care and get desensitized to them. When I was guard dogless, I could walk out my front door and there would be 5 coyotes just passing through. (also had a mountain lion once)
I don't understand the comment to "light the feeders". Your horses eat fine in the dark. They can run at a dead gallop in the pastures at night as easily as they do in the day.
It is useful to have some type of light available in the barn after dark but just for you.