Question about Hetairoi

[MeinPanzer]

MP, I would have figured the shield would have been adopted sometime after Antiochos III, at least that is my impression since Livy describes them as similar to the cataphracts at Magnesia.

It could have been under Antiochus III, but I don't see any reason why it would have - it could just as easily have emerged before him. All Livy's description tells us about the riders themselves is that they were quite heavily armoured; whether that includes shields or not is impossible to tell.

About the closest image I've seen to a Seleukid hetairos would be the cavalryman on the back of a few of Seleukos II's coins said to represent Seleukos himself (the diadem is the give away). No shield, but on one of them appears to be those nifty segmented boots we see on a few statuettes.

I don't think I'm familiar with this coin. The only coin of Seleucus II that I'm familiar with that shows anything close to a cavalryman are issues with a horse standing above a shield emblazoned with the Seleucid anchor (which, again, could be taken as indicating that royal cavalrymen carried shields). Could you post a picture of it? I also don't really understand what you mean by segmented boots. You mean like laminated armour covering only part of the leg?