The Sarmatians were distant kin of the much earlier Scythians. After a period of contention with the Roman Empire, they were defeated and their highly effective cavalry were press-ganged into service as auxiliaries to the Roman army -- but in far-flung areas, out of contact with the rest of their people, who were resettled in places. The Sarmatian cavalry saw most of its service in Britain, and some attribute the practice of the medieval jousts to the Sarmatian roots (I doubt this, btw). Ethnically and linguistically, they effectively ceased to exist by the time the Empire fell apart in the west. Another related people were the Alans.
And to walk down “historical movie road” the Sarmatian cavalry played a major part in the movie “King Arthur” that stared Clive Owen as Arturius, a Roman cavalry commander.
Lancelot: [voiceover] By 300 AD, the Roman Empire extended from Arabia to Britain. But they wanted more. More land. More peoples loyal and subservient to Rome. But no people so important as the powerful Sarmatians to the east. Thousands died on that field. And when the smoke cleared on the fourth day, the only Sarmatian soldiers left alive were members of the decimated but legendary cavalry. The Romans, impressed by their bravery and horsemanship, spared their lives. In exchange, these ...
See: https://www.imdb.com/title/tt0349683/?ref_=nv_sr_2