Great Old One

The Old Ones, sometimes known as the Great Old Ones, were beings from before the universe, according to the Doctor. (NA: White Darkness, All-Consuming Fire, MA: Millennial Rites)

Overview
The Great Old Ones were unimaginably ancient creatures that were preserved in Humanity's race memory as their worst nightmares. They were conceived as creatures that spread evil, chaos and destruction wherever they went. (BFA: The Roof of the World) The sensors of a TARDIS noted the body of a Great Old One appeared as a massive quantity of energy which was the equivalent of energy from twenty billion nuclear bombs. (MA: Millennial Rites) They were known to possess physical bodies which they sought to return to and had a third part of their form which existed in the outer dimensional planes which some believed was where they originated from originally. (NA: White Darkness) They were known to be one of a number of creatures that had adapted in a way that made them capable of perceiving the higher dimensions. (PDA: Tomb of Valdemar)

Quantum mnemonics was known to be the language of the Old Ones, as they represented physical laws of the Universe that preceded the current one. It was considered the Old Ones' equivalent of Block Transfer Computations and allowed them to rewrite reality on the most fundamental of levels. (MA: Millennial Rites)

It was known that the Great Old Ones were incapable of being killed in the normal dimension. They also knew of the Time Lords and considered them a threat. (NA: White Darkness

History
The beings that became known as the Old Ones did not originate from this Universe but the one before it. They were originally a powerful race of beings who were the equivalent of the Time Lords in their native space. However, when the Universe ended, the Old Ones shunted themselves into a paralle dimension which collapsed seconds later into the new Universe. This allowed them to pass into the new Universe which had a different law of physics and they discovered that they had gained god-like powers which allowed them to manipulate the new Universe they now existed in. (MA: Millennial Rites)

They were worshipped as gods on Earth during the age of the Silurians who knew them as gargantuan entities that once ruled the planet and were known to a cult of the Shobogans. According to the legends, it was believed that Azathoth was the weakest of their kind (NA: All-Consuming Fire)

Ancient carvings dating at least fifteen million years ago show that the Old Ones ruled everywhere before they died out and went into hibernation after fleeing from the predators with a touch of decay. They managed to escape from the great cosmic disaster in order to flee into the Time Vortex where they became discorporate entities thus leaving their bodies behind on Earth.

Their minds have sought to return to their bodies when the stars were correctly aligned as the tidal forces of the stellar masses in conjunction can bring about a sufficient tear in the space-time continuum which can bring about a pathway through which they can reunite with their bodies. However, as the stars were constantly expanding and contracting, they never returned to the same configuration as the time when the Old Ones went into hibernation. Thus, they required the assistance of their followers to attract the attentions of their bodies autonomic insticts and allow them to chance to compensate. As they were unable to affect the world physically, they used the influence of their powerful minds to achieve their goals. (NA: White Darkness)

At one point they attempted to enter into the higher dimensions by using the Palace of the Old Ones to power a gateway. One of their number, known as Valdemar, discovered that his kind were unprepared for the perceptions of this new reality and were being driven mad by it. This breach would have destroyed the universe had Valdemar not sacrificed himself to seal the breach. Legends would spread across the galaxy that he was a dark god that nearly destroyed the Old Ones though the truth was far different. The mystery of Valdemar and the Old Ones were later known as number six in the great mysteries of the universe. (BBC: Tomb of Valdemar)

The Pharoah Amenhotep discovered them at one time on Earth and attempted to harness their power for himself. However, he came to realise that they were beyond his control and lured them into a white pyramid with a structure of infinite reflections where the creature's evil were to be trapped forever. This trap succeeded and kept the world safe from their evil. (BFA: The Roof of the World)

List of Old Ones

 * Azathoth
 * Cthulhu
 * Dagon
 * Hastur the Unspeakable
 * Several Lloigor, including the Animus
 * Nyarlathotep
 * Shub-Niggurath
 * Yog-Sothoth
 * Magog
 * Nah
 * Raag
 * Rok
 * Valdemar

Possible Old Ones

 * Though not an Old One itself, the Nestene Consciousness is the child of Shub-Niggurath. (PDA: Synthespians™)
 * Though not explicitly described as such, the god-like Golgoth seems very clearly reminiscent of the entities of the Mythos. (DWM: The Cybermen: The Dark Flame)
 * An unknown Old One was said to be within the ship Infinite as a power source. (DW: The Infinite Quest)
 * Given its immense power, its huge body and its Pre-Universe origin, it is very possible that the character of The Beast is an Old One. It's equally probable however that The Beast is something even older and more terrible than the Great Old Ones.

Behind the Scenes
The Old Ones originated out of the Cthulhu Mythos, a shared universe based on the writing of H.P. Lovcraft and to an extent, some of his contemporaries. A few characters and concepts in televised Doctor Who, such as Sutekh (though based on an actual ancient Egyptian god) and the Fendahl, had shown the apparent influence of the Mythos. However, not until the novels White Darkness, All-Consuming Fire and Millennial Rites did beings explicitly referred to as Old Ones appear in fiction set in the Doctor Who Universe.

The latter two books took known Doctor Who enemies, such as the Animus and retconned them into having links to the Mythos. The Taking of Planet 5 revealed that the Cthulhu Mythos also existed in the form of fiction in the Doctor Who Universe. Though the established Cthulhu Mythos and the Doctor Who version have some differences (the idea of the Old Ones originating before the universe originates in Doctor Who novels, for example), stories set in the Mythos frequently contradicts itself.