Demon

A Demon is an evil creature in the mythology of the Humans. They are generally depicted as being humanoids with horns and magical powers.

The Dæmons are a race of advanced aliens from Dæmos. During their travels on Earth, they probably influenced the mythology of the native Humans. (DW: The Dæmons)

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

The Destroyer was an entity from another universe who unwillingly served Morgaine. The Destroyer had a vulnerability to silver which enabled Morgaine to chain him with manacles made of it. When shot with silver bullets fired by Brigadier Lethbridge-Stewart, the Destroyer was apparently killed. (DW: Battlefield)

The Beast and Abaddon are both ancient individuals who were sealed away to protect the universe (in Krop Tor and the Cardiff Rift, respectively). (DW: The Satan Pit, TW: End of Days)

The Destroyer, The Beast and Abaddon all share similarities. All three fit the depiction of demons and the Destroyer said he wished to "devour" Earth, while Abaddon absorbed life energy. Abaddon is said to be the son of the Beast.

References

• In the court of King John I, the Doctor, Tegan and Turlough found themselves mistaken for demons due to their "magical transportation". (DW: The King's Demons)
• Demnos, rearranged around, spells demons. (DW: The Masque of Mandragora)