These were modern books. As an example, the Time/Life book "Imperial Rome" has a six-page section called Roman Supremacy In The Siege, and specifically discusses Julius Caesar's siege of Avaricum in 52 BC. It discusses "onagers" in numerous passages.
I've also read a number of novels which refer to the smaller catapults as onagers, in a wide variety of time periods. I know novels aren't the best reference, but the term just seemed so prevalent, I was surprised to hear it should only be used in the late Empire.