Although most closely associated with the Byzantines, "Cataphract" (which IIRC simply means "armored" -- e.g., there were so-called "cataphracted" galleys) is somewhat generic, refering to partially armored cavalry equipped with both bows and shock arms (swords and/or spears). The earliest application of the term is used re: the Sassanid Persians in the 3rd century CE, although, again, it most often refers to the Byzantines. Byzantine cataphracts were the best cavalry in the world at the time -- professionals, capable of dealing with both Western European shock cavalry and Eastern horse archers. I'm still playing around with stats, but I believe they should be slightly better in both attack and defense than the knight, and have some missile fire capacity, as well as an additional hit-point due to the rarity of other, contemporary, professional armies.
-Oz
PS closest relative Civ would have to be the Romans, although this is in some ways a stretch, as the Byzantine Empire was based around a Hellenized, far more urban culture than the Western Roman Empire. Language and religion soon separated them from their western brethren; their cultural contacts to the east made them a cultural hybrid; and their military was influenced by both the eastern horse-archers and the Vikings (the ones who turned east were known as Varangians, and travelled down the riverways of Russia, and became the palace guard at Constantinople).
-Oz