In the Late 3rd century BC, the Palintones built a huge stone-throwing catapult (called "ballista" by the Romans.) This was a huge problem for Roman armies, seeing as the ballista could easily destroy Roman encampments and fortifications in minimal time from a very long distance. In order to counter this threat, the Romans engineered the onager. Named after a pig that throws stones behind it when it runs, the onager was smaller, lighter, and easier to construct than the ballista because, in essence, it was half the machine. The frame of the onager was composed of thick rectangular pieces of wood laid flat on the ground. Through each side of the frame was bored two holes through which ran the skeins of rope. The ropes were held in place by a washer and counterplate. In the middle of the sinew ropes stuck a single arm that ended in a cup or a sling fitted for a stone. The arm was cranked down with a lever, further torsioning the skein, and was held in place by a ratchet and pawl. When released, the arm would snap forward into a supported upright which halted the arm and drove the shot towards its intended target.
This form of siege engine, though smaller and less complex than the ballista, proved just as effective. The onager was difficult to move, and was replaced by more advanced forms of catapult like the trebuchet.