The Romans, finding
the ballista difficult to construct, simplified the design and created
the onager. It had one arm instead of two, and is what is most
commonly identified as a catapult today.
http://members.lycos.nl/onager/OnagerPic.jpg
During the Dark Ages, the French were able to re-invent the
onager, and they called it the mangonneau, and it became known in
England as the mangonel.