1 Armoured knight: I see that the dragon is on wheels, but what makes it move? There are no horses or oxen.
2 Shakespeare: We have a brace of turnspit dogs below the floor, to turn the wheels.
3 Armoured knight: Aha! Most cunning!
4 Mercutio: I would have just said it was magic.
4 Shakespeare: Do you want to get burnt as a witch?

Turnspit dogs were a type of dog bred specially to work as a labouring animal. Specifically, their job was to run/walk inside a wheel—somewhat like a giant hamster wheel—to provide motive power for turning a turnspit, also known as a roasting jack. The turnspit was a device for rotating meat on a spit, over a fire for roasting. Turnspit dogs seem to have existed in England from at least the mid-16th century to the 19th century.

Presumably with the rise of automation around the Industrial Revolution, the role of dogs in this job was supplanted by machinery. Turnspit dogs seem to have then died out as a breed or type. They were short and stocky dogs, with short legs, allowing them to enter and walk on the wheels. Probably not attractive dogs, thus sealing their fate.