Aren't the ponies usually nubile young ladies?
It's a fairly light form of bondage and submission, to harness a beautiful girl with leather, comb out her long hair and get her trotting round the room with perky breasts bobbing...
I first heard/ saw it in
Secretary. About the only time spanking has ever turned me on was when James Spader was administering it. I nearly slid off my chair.