When did cohors equitata milliaria come into existence?

[musterion]

Thanks,  I was trying to figure out if the chilliarchos (literally, ruler of 1000) in Acts 22 was in command of such a unit.  When Paul was taken to Caesarea he was surrounded by 200 infantry, 70 horseman, 200 spearmen, and I thought the it would have to be a big cohort to have this many.  As I recall, Judea was garrisoned by auxilliaries and not by frontline legions.