When you ask people to describe an Onager, they will always make the big mistake that an onager has a sort of spoon on the end of the arm. This ofcourse is caused by all the Hollywood movies in which the onager is present, always having a spoon or bucket. This page is all about the wrong ideas people and artists have of catapults.
If you look into a dictionairy and search up the word onager, you'll probably get the following explanation: "Onager: also known as mangonel, trebuchet, ballista, catapult." This is wrong. An onager and a ballista have torsion bundles to throw their projectile, a trebuchet has a counterweight. Also, a ballista has two throwing arms, an onager only one. Another mistake is that between the scorpion, the onager and the mangonel. The onager and the mangonel are exactly the same siege engine, the word onager is actually a nickname coming from the kicking action when fired, resembling a wild donkey. The name scorpion, which is sometimes used to describe an onager, comes from the appearance of the throwing arm. With the pin on the end of the arm, it resembles the "upraised sting of a scorpion". The real scorpion is a siege engine similar to a ballista, only much smaller and more mobile.
People tend to describe an onager as having a spoon or bucket at the end of the throwing arm. This is wrong. The roman engineers knew that if they added a sling to the end of the arm, the onager would throw its projectiles further. The real appearance of the onager will probably never be known to the public, because every movie featuring an onager depicts it having a spoon. Another big mistake in the looks of an onager is that some artists prefer it to have a big bow attached to the frame. They think that this, together with a spoon, is the form the onager should be in. Most artists understand that an onager has to have a torsion bundle, but some artists draw it so that it is twisted the wrong way, making the onager unable to work. Almost every artist draws wheels on the onager. This is also wrong, because wheels only negatively affect the range. They only aid in easier transportation, but should be taken of when fired.