The onager was a post-classical Roman siege engine, which derived its name from the kicking action of the machine, similar to that of an onager (wild ass). It is a type of catapult that uses torsional pressure, generally from twisted rope, to store energy for the shot.

The onager consisted of a frame placed on the ground to which a vertical frame of solid timber was rigidly fixed at its front end; through the vertical frame ran an axle, which had a single stout spoke. On the extremity of the spoke was a sling used to launch a projectile.

In action the spoke was forced down, against the tension of twisted ropes or other springs, by a windlass, and then suddenly released. The spoke thus kicked the crosspiece of the vertical frame, and the projectile at its extreme end was shot forward.

The onagers of the Roman Empire were mainly used for sieging forts or settlements. They would often be armed with huge stones or rocks that could be covered with a flammable substance and set alight.

In the Middle Ages (recorded from around 1200) a less powerful version of the onager was used that held the projectile in a fixed bowl instead of a sling. This was so that many small projectiles could be thrown, as opposed to one large one. This engine was sometimes called the mangonel, although the same name may have been used for a variety of siege engines.