There’s a famous blasphemous piece of graffiti from an ancient Roman boys’ school on the Palatine Hill. It shows Alexamenos, apparently an early Christian, worshipping the crucified Christ — except that Christ is shown with a horse’s head. (In the next room, btw, there’s another piece of ancient graffiti saying that Alexamenos is faithful.)
I think I’ve figured out the point of the joke.
“Mannulus” is the Latin word for a small breed of cob horses or ponies from Gaul, suitable for drawing coaches.
Emmanuel is Alexamenos’ god.