Below, the same drum from outside: inside it, a strip of photos of the horse, each a moment later than the last.
Toys that make pictures move
Zoetrope
What happens when you look through the slits of the spinning drum?
The horse gallops.
Why? The same trick as the phenakistiscope, with a strip of pictures inside a drum, so several people can watch at once.
Picture: cmglee, CC BY-SA 4.0 · Wikipedia(opens in a new tab)