I am not even sure if any of the factions can build them. I've seen them in Byz armies though. If they can by build and not only hired, how?
Also what's your opinion on those guys? I thought they were pretty good, but I recently defeated soundly a whole bunch of them with my Avar Nobles. What is their usage? Are they just like Hobilars/Saharan Cavalry/Mounted Sergeants?