Onager

The onager was a Roman torsion-powered siege engine, a type of catapult typically fitted with a bowl, bucket or sling at the end of its throwing arm to hurl stones at fortifications or enemy formations. It is first documented in 353 AD by the historian Ammianus Marcellinus, who equated onagers with the earlier scorpion, though the device may have originated as far back as the third century BC. Its name comes from the kicking action of the machine as it threw its projectile, likened to the kick of the onager, the wild ass native to the eastern Roman Empire; historians have sometimes confused the onager with the mangonel, a later traction trebuchet that replaced torsion siege engines by the sixth century AD.