The onager was invented to replace the complicated ballista(SKORPION). The onager is much easier to use and required fewer men to operate. It had only one arm, which meant it needed only one torsion spring, so it didn’t have as many moving parts to worry about. This means that more soldiers could learn to use it. It did not have the range that the ballista had, though.
The Romans would include an onager with a Centuria, which is a group of about one hundred soldiers. It took twelve soldiers to operate the onager.
It is a torsion engine that derives it’s power from a large skein of twisted ropes. The ropes need to be strong and elastic, so often were made from hair or sinew.