The Onager is a Roman-style catapult on a wooden frame which launches its projectile using a sling that is forced down against high tension and then released. The name of the weapon means "wild donkey," and has been attributed to the mechanism's visual similarity to the animal's kicking hind hooves.

The Atlanteans made great use of Onagers. It was their most effective artillery for besieging a city and for firing at high-density troop formations.