I've been reading up about the cataphracts since I find byzantine military history enthralling, and I read that they only used the selectively bred "Nisean" horse for the cataphract units. Of course, this use predates the Byzantines, with the Parthians, Archimaemenid Persians and Seleucids also having used this breed to create their Cataphract units. The last of the Niseans were made extinct by the sacking of Constantinople in the fourth crusade.
It was said to have
"not the slender Arabian head of the Luristan Culture but a more robust one that was characteristic of the great warhorse". Another is that the were "tall and swift, and color adorned his sides".
They had legendary status in antiquity - Wikipedia says
"During the reign of Darius, Nisean horses were bred from Armenia to Sogdiana. The Nisean horse was so sought after, that the Greeks (mainly, the Spartans) imported Nisean horses and bred them to their native stock, and many nomadic tribes, (such as the Scythians) in and around the Persian Empire also imported, captured, or stole Nisean horses."
A.T. Olmstead, in his History of the Persian Empire says that pure white Niseans were the horses of kings and, in myth, gods. Cyrus the Great was so distraught when one of his stallions was drowned while crossing a river, he had the river where the horse was drowned drained. He did not believe that anything that could kill a horse so beautiful.
That being said, I can't find any uses of Cataphracts being used in the Byzantine empire, or in any other context past around 1100. Did the Nisean horse lose its exclusivity and value with the advent of the selectively bred western heavy cavalry? From the limited reading I've done, early knights of Feudal Europe also used selectively bred horses in combat that were stockier, slightly slower but far hardier than the average horse - these characteristics are akin to the Nisean. Would the Nisean still maintain its superiority in, say, 1200?
Or does its legendary status only hold due to the fact that hellenic and eastern horses were traditionally smaller and slighter, so the Nisean stood out completely? Would it still be a behemoth in comparison to the Western European warhorses of the 13th century?
What is your source for Byzantine use of Nesaean horses? John Haldon, Ann Hyland, and others tend to stress the diversity of horse types used by the Byzantine army. After all, this is our rebranded "Roman" army, and Rome's cavalry was multinational, even multicontinental. Rome acquired such a diversity of horses often through incorporation of cavalry units from conquered regions: Gaul, Iberia, Greece, Syria, North Africa, the Balkans. The focus for breeding would seem to have been towards size rather than "purity," since the average withers height of Roman horses increased 10 cm (to 13.1 hands) over Iron Age precedents. However, that applies to all horses, not just warhorses; military use probably preferred the larger side of average. Romans apparently sought out Scythian horses to interbreed for size.
Under Byzantine rule, horse breeding and acquisition centered in Cappadocia, Phyrgia (western Turkey), and Syria. Especially in later centuries, this would have included Arabian stock (there is some archaeological evidence for Arabian horses in Byzantine armies as well) as well as a type of Central Asian horse noted for its difficulty to control--the opposite of the Nesaean.
Vedat Onar et al., based on archaeological evidence from within the boundaries of medieval Constantinople, suggest that Byzantine horses carried on in the size range of Roman horses, especially warhorses: 13-15 hh. Hyland agrees (and points to Byzantine horses in Sicily as one source of inspiration for crossbreeding horses in the Latin West to increase their size).
We do have some evidence that in the 13th century, quality horses were still being bred in Persia and traded--except, they went east instead of west. Marco Polo writes:
In this country of Persia there is a great supply of fine horses; and people take them to India for sale, for they are horses of great price, a single one being worth as much of their money as is equal to 200 livres Tournois; some will be more, some less, according to the quality. (source and note that "India" can be generic medieval for "the exotic East")
So I'm curious where you've read that Byzantine armies preferred Nisean/Nesaean horses, and where you saw that they went extinct in Constantinople in 1204?