They were Iranian nomads with a language similar to that of the Scythians. Herodotus calls them Sauromatae and thinks they are descended from the union of Scythian men and Amazon women.
He says that the women go out hunting on horseback, with or without their husbands, go to war, and wear the same clothes as the men. He says that one of their marriage customs is that a woman cannot marry until she has killed a male enemy. (Book 4, chapters 110-117)
Well known Amazon Woman:

Not sayin’ it was aliens, but maybe it was Somemartians.
The term “Scythian” is as meaningless as “barbarian” — scythian being the generic term the Greeks gave to north-western barbarians (”barbarians” being the generic Greek term for non-Greeks!)
It’s as funny as the Zhou dynasty Chinese giving names to the groups of people around them (and we don’t know what these people called themselves as the Chinese/Huaxia people had writing and the others didn’t) - the “northern barbarians”, the “western barbarians”, the “southern barbarians” :)
Anyway, so the “Scyths” were just the generic terms for a whole buncha peoples - whether Iranian or Slavic or other
The Sarmatians are a later group of different Alani (Aryani) tribes who were definitively Irani speaking