Reference:
William H. McNeill, The Rise of the West: A History of the Human
Community
The Sassanian rulers financed these armored fighting units by assigning them each an area which was responsible for providing their armor and horse and supporting them while they were trained. This arrangement led to the feudal social structure with the military elite at the top and the peasant-serfs at the bottom.
Later the system of armored knighthood spread to the steppes north of the Black Sea and on into Europe. The Sarmatians, an Iranian-language-speaking people, displaced the Scythians in what is now south Russia and the Ukraine.
The realm of the Sarmatians extended from the Han Emprie in the east to the Roman Empire in the west. From the Chinese, among other things, the Sarmatians adopted the dragon motif.
In the third century A.D. the Sarmatians fought the Romans near the mouth of the Danube River on the Black Sea. The Sarmatians lost the battle but they so impressed the Romans with their fighting prowess that the terms of the peace called for six thousand Sarmatian warriors and their horses to join the Roman army. The Emperor Marcus Aurelius sent 5500 of these Sarmatians to the northern border of the Roman Empire in Britain to guard it against attacks by the Celts.
Physical artifacts of this Sarmatian force have been found, including retirement villages. There are other things which may be cultural artifacts of the Sarmatian presence. There is a good chance that Celtic dragon image came from Sarmatian sources. The really intriguing possibility is a link between the Arthurian legends and the Sarmatians. In the religion of the Sarmatian the altar was a sword embedded in a stone. Arthur's father was Uther Pendragon. Pendragon means "dragon's head" and refers to the dragon-head symbol on a shield. The evidence suggests that the Sarmatians with their armor created a military caste that survived several centuries and provided the leadership in the early feudal era.