The onager is a area-of-effect siege unit that is trained from the siege workshop or constructed in the field by legionaries.
Since legionaries are added in another patch, their ability to build onagers isn't added here.
Splash range of 4, 80 crush.
Direct attack of 40 crush
The onager has much less health than a catapult, and also costs less. It may still be used versus buildings but it is most effective against soldiers.