Carthage does get to recruit onagers. IMHO, these are best used when attacking as you can move the onagers forward just enough to bring the enemy under fire and not precipitate a major engagement, at first. On the defensive, you might get several shots in before enemy units get too close to risk friendly fire. You'd have to be extremely lucky to hit the unit you want to. Not worth having one or more units slots taken up by units that will have little impact on the battle.