Roman Onager

The onager was a single-armed torsion catapult used by the Roman military from the late Empire onward, named for the wild ass (onager in Latin) for the way its firing arm kicked upward violently, resembling the animal's buck. Unlike the twin-armed ballista, the onager used a single vertical throwing arm anchored in a thick skein of twisted rope, drawn down and released to hurl stones or other projectiles in a high, arcing trajectory well suited to attacking fortified positions from a distance rather than engaging troops directly. The Roman military writer Ammianus Marcellinus described the weapon's operation and its adoption as a standard Roman siege engine in the fourth century AD, by which point it had largely supplanted earlier two-armed stone-throwers in Roman service. The onager's design and its high-arc throwing principle directly influenced medieval siege engines, including the later and more powerful counterweight trebuchet that would eventually surpass it in range and payload.