Demon

A Demon was an evil creature in the mythology of the humans, although similar deities appeared in the religions of races such as the Kaleds and the Draconians. (TV: The Satan Pit) They were generally depicted as being humanoids, often with horns.

Self-described demons
The Destroyer was an entity from another universe who unwillingly served Morgaine. The Destroyer was vulnerable 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. (TV: Battlefield)

The Beast called himself the influence for all devils in all mythologies. (TV: The Impossible Planet)

The Beast's son Abaddon was an ancient being sealed behind the Cardiff Rift. (TV: End of Days) Abaddon also fought another demon who resembled him, Pwccm. (PROSE: The Twilight Streets)

Evil entities
The Mara had some of the powers of demons, include the ability to possess bodies. (TV: Kinda, Snakedance)

Fenric had many of the qualities associated with demons. (TV: The Curse of Fenric)

The Master was seen to humans as a demon when his form was comprised solely of energy. He used demonic powers such as possession to find new hosts for his dying form. (PROSE: Prologue)

Similar beings
The Dæmons were a race of advanced aliens from Dæmos. During their travels on Earth, they probably influenced the mythology of the native humans. (TV: The Dæmons) They themselves had a figure in their mythology similar to the Beast. (TV: The Satan Pit)

Melanicus, a being resembling the medieval conception of a demon, stole the Event Synthesizer and briefly warped the universe into his personal toy. (COMIC: The Tides of Time)

Marchosias, a gestalt manifestation of the Psionovores, took the form of an eight-foot tall bright red demon when "summoned" as an agent of Lucifer by Brigham Elisha Dashwood III and the Hellfire Club. (AUDIO: Minuet in Hell)

An entity from a Shadow Dimension was hired by the alien Hornets to trap the Doctor and drain his mind of all his knowledge. The entity's appearance and frequent need to drain the life force of humans to sustain itself made the Fourth Doctor call it a demon. (AUDIO: Demon Quest)