Same with the Weeping Angels, E-Space Vampires (Or at least the great one), Judoon, Jagaroth, Osirians, Vashta Narada etc. to add a few. The Doctor usually fills in the companion-and also the audience-on what they are. Of course he's been taken by surprise a few times by the Master, either through his use of disguises (No matter how flimsy) or appearing in a body unknown to him (Yana, Missy, O, briefly the "Bruce" Master).
There are of course ones that are 'fresh' to him, at first-Daleks and Cybermen, for example. He didn't know anything about them when he first met them. And even after many battles with the Daleks, he didn't know anything about Davros until "Genesis of the Daleks".