B is for... Beast

An incredibly ancient being which was the embodiment of Evil. The Beast claimed to predate Time itself, which the Doctor found hard to swallow.
Millennia ago, the Beast was captured by the Disciples of the Light and imprisoned at the heart of a barren planet, Krop Tor. This was placed in geostationary orbit around a Black Hole. The only way it could escape would be to smash a pair of vases in the cavern in which its was held captive. However, destroying these would plunge the planet into the Black Hole.
In appearance, the Beast was an enormous, red-skinned horned being, with a skull-like face. It could breathe fire. The Doctor speculated that it may have been the origin for Devil figures on many, many planets.
Unable to flee physically, the Beast planned to engineer an escape for its mind. It generated a power signal of incredible magnitude, as well as a gravity funnel that would allow any curious visitors to safely reach the planet.
In the year 43K2.1, an expedition arrived from Earth to investigate the power source. The Beast possessed one of the crew - archaeologist Toby Zed. It then turned their peaceful Ood servants against the humans so that they would be forced to flee - carrying the Beast's mind away within Toby's body.
At times its control over him would manifest itself physically - his eyes glowing red and strange markings would appear on his flesh.

The Doctor descended into the cavern where the Beast was held, and he quickly realised that something was wrong. The creature he saw before him was pure animal instinct, with no hint of its considerable intelligence. Thus he worked out that it was abandoning its physical shell, but its essence was somewhere else.
He smashed the vases and so destroyed the Beast's body as the planet fell into the Black Hole. The creature revealed itself on the escape ship as the markings appeared on Toby's face and he began ranting and breathing flames. Rose used a bolt gun to shatter the window, unlocking Toby's seat belt at the same time. He was sucked out of the cockpit, and so the Beast's mind also perished in the Black Hole.

Voiced by: Gabriel Woolf. Played by: Will Thorp (Toby Zed form). Appearances: The Impossible Planet / The Satan Pit (2006).

• Gabriel Woolf had previously voiced the Osiran Sutekh in Pyramids of Mars. The Doctor had thought that he might also have inspired the Satan myth.
• Back in 1972, it was the Daemons whom the Doctor had thought responsible for the horned devil legends - on Earth at least.
• Here the Doctor blames the Beast for possibly being behind the devil characters of the Kaleds and the Draconians amongst others - including the Daemons themselves, oddly enough.
• The Great Intelligence claimed on Trenzalore that the Doctor himself would ultimately become the Beast - though this would evolve from the Valeyard.
• The Beast was survived, briefly, by a son - Abbadon. He fell foul of Captain Jack in the Series One finale of Torchwood - End of Days. 
• Rather bizarrely, in a segment of the Sarah Jane Adventures Files, in which computer Mr Smith provides the equivalent of a clips show to Luke, it is stated that the turns-out-he's-not-that-bad-really criminal Androvax is higher on the Judoon most-wanted list than the Beast, who's Mr Evil Incarnate.