This model of an onager was probably made at the request of Napoléon III by his ordnance officer Jean-Baptiste Auguste Verchère de Reffye. The onager was a Roman war machine used for firing projectiles over long distances. It was used during sieges to break through enemy walls.
Pulled along by animals when it needed to be transported along siege lines, it had two parts, an articulated arm for throwing and a buffer to break the fall of the arm.

Model of an onager