The Roman Onager was the main artillery device of the Roman army. It was named after the Asian Wild Ass because they both have a tendancy to "kick" when they go off.

Oftentimes, an Onager would be fitted with a sling on the end of the arm instead of a cup or bucket. The sling will give greater range to a heavier projectile, but can only hurl one rock at a time. The bucket could be filled with lots of rocks at once - an effective anti-personnel device.

The kinds of things the Romans hurled includes heavy rocks and clay pots filled with pitch and tar- on fire.

Onagers are sometimes called a "Mangonel", but Mangonel literally translates as "War Machine" - a category that also includes ballistae and trebuchets.

The Onagers were used from early Roman times right up to the medieval period alongside the trebuchets. They lost their advantage to the cannons and gunpowder in the 13th century.