The Doctor and Ida in “The Satan Pit”

You get representations of the horned beast right across the Universe. In the myths and legends of a million worlds: Earth, Draconia, Vel Consadine, Dæmos, the Kaled god of war. It's the same image, over and over again. Maybe that idea came from somewhere, bleeding through, a thought at the back of every sentient mind.

Emanating from here?

Could be.

But if this is the original, does that make it real? Does that make it the actual Devil?

Well, if that's what you want to believe. Maybe that's what the Devil is in the end: an idea.