Once when I was around 15 I was walking around where I used to live, which was deep in English countryside. I heard a horse approaching me from behind me, which is common as there was a polo club nearby. When I felt the horse was close enough, I turned to let it pass. There was nothing there. The sound continued to get louder and quieter, as if the horse was passing me, but it was not there. There is nowhere it could've been. Creeped me out.