Demon

As well as the alien Dæmons and other demonic beings, literal demons apparently exist.

One, Melanicus, stole the Event Synthesizer and briefly warped the Universe into his personal toy. (DWM: The Tides of Tide)

Another, The Destroyer, unwillingly served Morgaine. The Destroyer found silver powerful over him, enabling Morgaine to chain him with manacles made of it. When shot with silver bullets fired by Brigaider Lethbridge-Stewart, the Destroyed apparently died. (DW: Battlefield)

The Cardiff Rift bound Abaddon, a being named as a demon in the Bible. (TW: End of Days).

Other references

 * In the court of King John I, The Doctor, Tegan and Turlough found themselves once mistaken for demons. (DW: The King's Demons)