Our Parthian expert has been caught up in real life and so he can't answer this question. And unfortunately, I'm not a Parthian expert myself. However, with what knowledge I have I can tell you that the late bodyguard is probably 1st century AD. It's inclusion in the Battle of Carrhae could be debated quite strongly and the one defense is that we don't truly know exactly when such incredibly heavy cavalry showed up.

What that means for EB2, I don't know. We haven't gotten that far yet. Although any arguments you could make for or against the Imperial Roman units would be the same.