They were called sagittarii, and they were raised from the auxiliary. I know for a fact that Tacitus, Procopius, and the Notitia Dignitatum mention them, with emphasis on mounted archers. And they're on Trajan's Column.

I don't know why they haven't appeared to you yet. I would imagine that they receive more emphasis after Carrhae, when Crassus was absolutely routed by Parthian archers (not fielding any on his own part), but I do not know much about that period. It is also potentially because they weren't legion units; auxiliary are de-emphasized in some accounts despite being often more meaningful than the legion.

It may be that you haven't come across them yet because you are only studying Roman legion training and reforms. Since the archers were in the auxiliary, you wouldn't see references to archers unless you looked outside legion forces.