Not a bad idea at all. Is there already a cat for horse troughs? If not could your cat be for "public equine utilities"?
Off topic, and not intending to hijack your thread, but I had an idea a while back that would encompass hitching posts. I was thinking along the lines of byegone street furniture. Any original surviving object that would have been as unremarkable in a previous age as a pedestrian street crossing is now: horse hitching/drinking posts, public water pumps, gas street light infrastructure etc. The problem being that some/all of these already have their own categories. So maybe it's an idea whose time has passed.