Great One (White Darkness)

According to some accounts, the beings remembered in occult Earth lore as the Great Old Ones were a specific race of powerful interdimensional beings, known in their prime as the Great Ones, who held dominion over a number of planets around 15 million years before the 20th century. The Seventh Doctor prevented one of these Great Old Ones, (PROSE: White Darkness) Cthulhu, (PROSE: All-Consuming Fire) from rising again in 1915 Haiti. (PROSE: White Darkness)

History
According to the Seventh Doctor, the Great Old Ones "originally came from some other universe, other dimension – one of the outer planes, most likely – and part of their being still reside[d] there". Among other planets, all of which had similar carvings as archaeological evidence of the Old Ones' presence, they were one of the groups which claimed mastery over the Earth in prehistoric times, having come there from a distant galaxy.

There, they coexisted with the "Star People" and the "reptile men" in a shared, advanced civilisation. However, over time, the reptile men "departed from the face of the world" (along with their augmented apes) and the Star People fled back whence they came after their genetically-engineered slave caste of servitors turned on them. Eventually, only the Great Ones were left. They were then besieged by "the unarmed predators who hunted with one huge fang and the touch of decay", and decided to hide their physical forms underground while shunting their consciousness into the Time Vortex, where it rode on the Time Winds, awaiting stellar alignments which would allow the reunification of mind and body, and able to telepathically influence cultists to make the necessary preparations for such an event.

One such Old One buried its body beneath the island of Hispaniola. Its body's natural telepathic instincts became active for several days in 1915, seeking to control humans into performing rituals that would summon his full consciousness back into his physical form; however, he was thwarted by the Seventh Doctor. (PROSE: White Darkness)

Later accounts
According to later accounts, the Old One encountered by the Doctor in Haiti, Cthulhu, was a member of a very different understanding of the "Great Old Ones", as a group of beings who had survived the death of the universe before the Big Bang, and had gained godlike powers in the new reality; he was thus the equal of the likes of Nyarlathotep and Yog-Sothoth. Though described in some accounts as merely a group of beings who had found a way to survive together, (PROSE: All-Consuming Fire) other accounts of these Great Old Ones or elder gods speculated that they may have been an earlier race of Time Lords. (PROSE: Millennial Rites, Divided Loyalties) Another member of this race was Lloigor, otherwise known as the Animus of Vortis; (PROSE: Millennial Rites, All-Consuming Fire) the Second Doctor's description of the Animus's kind as "the Old Ones" was midway between the two accounts, both referring to them as a specific race of conquerors with bounded ability, but also as survivors from another universe. (PROSE: Twilight of the Gods) "The Animus is one of an incredibly ancient race sometimes called the "Old Ones". They are beings of great mental power, who regard all other life forms as inferior to themselves and suitable only to serve them or to be incorporated into their own substance as they grow and spread their influence."

- Second Doctor (PROSE: Twilight of the Gods)

Behind the scenes
David A. McIntee's depiction of the "Great Old Ones" in White Darkness as a race of powerful, but ultimately non-divine, "ancient aliens" is actually closer to H. P. Lovecraft's usage of the term in than later usage of the term in works by Craig Hinton, Gary Russell and Andy Lane to refer to various spins on the idea of a panthon of eldritch beings from the pre-universe, which established a lasting confusion in Doctor Who/Cthulhu Mythos crossovers between Cthulhu's people the Great Old Ones, and the entities more commonly referred to in Lovecraftian lore as "the Outer Gods", "the Other Gods" or "the Elder Gods". In fact, in Lovecraft's depiction, the Great Old Ones worshipped the Other Gods in much the same way that human cultists might worship the Great Old Ones.