Of course, some of the things that the Doctor outright derides about the Beast - being a supernatural evil, existing before time - contradicts parts of classic continuity, specifically the Seventh Doctor.
In
Battlefield, an alternative Arthurian universe breaches into Who!Earth's. The Doctor describes it as a universe where magic was the prevalent science (mentioning Clarke's Third Law - "any sufficiently advanced form of technology is indistinguishable from magic" - before saying that the reverse is also true). The Doctor is stated to one day become Merlin, and Morgaine summons
the Destroyer, a self-proclaimed demon.
More specifically, there's
The Curse of Fenric. The titular
Fenric is referred to by the Doctor as a consciousness of pure evil that existed before the Big Bang. "EVIL! Evil since the dawn of time!" (It's mentioned in the episode that Fenric is just a name allocated to this evil - "evil needs no name" - and in the novelisation, the Doctor allows that referring to Fenric as evil is a generalisation, before describing it as "out of balance").
Make up your mind, Professor.
My headcanon for this is that during the Last Great Time War, either these entities fled from what the Time Lords and Daleks were doing, or they were chased out. A TL;DR line from a fic I've considered writing about this idea:
"They looked upon the Oncoming Storm, and fled in terror."