Spray your cat down so it doesn't get hit by cars! And your horse, so... Yeah, not sure why you'd want a glowing horse. Scare the neighbours? Nightmare?
Who the eff wouldn't want a glowing horse? Maybe if you were a horse burglar or something. But otherwise ... I'd be all about that.
There's a road sign for this too; https://en.wikipedia.org/?title=Warning_sign#/media/File:Swe....
Sometimes talking about your horse to by-standing victims isn't enough - in these cases, searing the image of the horse onto their retina is your only option.