Cataphracts

"Catphract" is a term derived from the Greek "Kataphraktos", meaning "heavily armoured in scale/chain", and it is indeed a fitting description. The original cataphracts were Iranian nomadic tribes' nobles, who could afford superior armor for both themselves and their mounts. The Parthians were the ones who took this to an extreme and created the image of the archetypical cataphract.
Parthian cataphracts were armoured in scale corselets, scale and/or chain that covered much of the body, and banded armor to protect the arms, legs, and thighs. Their horses had a quilted garment with metal scales sewn into it to protect their body (and sometime the neck as well) and a plate to protect the face, sometimes accompanied by scale and/or mail. Armed with a 3 to 4 meter kontos (Greek: barge-pole) lance, composite bow, and mace/sword, the Parthian cataphract was an expensive and powerful unit, dreaded by all who fought them.
The cataphract was adopted widely, especially by the Hellenic settlers of Baktria and the Arche Seleukeia, who were defeated by the Parthians many times. Superior Greek technology was used to armor the cataphracts more heavily, and they won many battles. However, the Greek commanders did not quite know how to use the cataphracts; Greek heavy cavalry was supposed to turn the enemy formation and charge home into the flanks or rear of the unsuspecting foe. Steppe nomads and Parthians (who were descended from the nomads) used the lancers as a heavy core cavalry who worked with the horse archers to weaken the enemy through attrition and arrows before charging the weak points that had been created. Not to mention the fact that the Parthians were natural horsemen and had superior morale, elan, and discipline than their Greek counterparts. The Greek cataphracts were mostly a replacement for the Hetairoi, who had all but died out by that time.