Quote Originally Posted by MeinPanzer
Again, if it was so unwieldy and made the horseman so vulnerable, why would so many wealthy and aristocratic citizens of numerous states equip themselves that way?
You may have noticed I'm fairly specifically arguing against them having done that, or in any case in the way you claim...

That seems reasonable. Still, one would expect the number of Hetairoi to decline after the introduction of cataphracts if indeed they were drawn from the Companions.
Cataphracts kind of have their limitations. Anyway, I'm willing to accept the argument the catas were drawn from amongst the Persian-Iranian aristocracy - many of those guys were probably already familiar with the kontos to begin with, so it'd really just have been a question of altering their defensive gear and tactics.

Of course, there is Procopius's whole schtick about not pushing the cataphracts to charge too soon or else they literally die before they reach the enemy!
I think we can safely dismiss that one as poppycock. Still, the heat problem is real enough and in combination with the sheer inertia and mass all the armour imparted on the horse doubtless resulted in stamina and mobility problems. Catas seem to have been pretty sucky at pursuit for example (both at Magnesia and on several other occasions the Romans fought them under diverse standards) and tired quickly, which besides sheer logic would rather seem to underline the point. As straightforward battering rams and shock troops the catas were clearly quite excellent, but that seems to pretty much have been it (aside from those also carrying bows being doubtless very difficult for light-cavalry skirmishers to deal with) - they were a very specialized type of cavalry after all. The Seleucids apparently didn't get that one worked out fast enough, unlike the catas' original users.

Anyway, one can see why the less encumbered and therefore more agile Hetairoi would've been kept around on the side as well, as more flexible elite shock cavalry.

Note that I said Nisaean and other eastern horses.
Fair enough, but all I've read about it has said quite specifically the Chinese were interested in the Ferghanans. Which probably had something to do with the breed being found rather closer to them than the Nisean.

Actually, the only shielded Celtic cavalry I can conjure up off the top of my head are cavalry carrying thureoi. The only image of a Galatian cavalryman that I know of is from a Bithynian stele. None of his weaponry or equipment is shown but he wears a kilt and has long hair and a moustache. Celtic cavalry were probably equipped with large round shields, but I've never seen any sources for that (most sources which have been identified as "Gallic" in the past are in fact misidentified).
The thureos looks like it'd make a pretty decent cavalry shield, or starting point for the developement of one, you know...