Medieval people seem to have fantasised a great deal about upside down worlds, perhaps because their real world was so hierarchical. I first encountered these fantasies as a child, at Malvern Priory. In those days you could enter the choir stalls and lift the seats to see the misericords underneath. I particularly liked the scene of rats hanging a cat. Being carved in dark wood, underneath a seat, it doesn't photograph well, but this drawing captures it perfectly.
Of all the upside down fantasies I have encountered since, this one from the Pontifical of Guillaume Durand is my absolute favourite. It's done with such complete seriousness, and the little snail-of-prey is inspired. Pontificals were books of services used by bishops, so they would only ever have been used by a handful of people. I love the fact that today we can enter into what was originally a private joke between an illustrator and a couple of bishops.