The Beast was a sapient being from the Doctor Who Universe who was imprisoned in the planet Krop Tor supposedly "before time began". It had seemingly supernatural abilities, and was akin to (or possibly was) the Devil.
Physiology
The beast was humanoid in shape, several hundred feet tall, could spit fire, and had horns.
It also had the ability to possess mutiple hosts. At one point, it was in control of several dozen Ood.
It claimed to be older than the universe.
History
The Beast was imprisoned on a planet that was artificially made to be orbiting a black hole. It was indicated that before time itself a great battle was fought between the Beast and the Ancients, in which the Beast was imprisoned.
The Beast may or may not be allied with the Light and may or may not be the father of Abaddon.