Pre-universe

The Pre-Universe was the Universe, perhaps the most recent of many universes, that came immediately before Normal Space, the home universe of the Doctor. After the destruction of the Pre-Universe, some species found their way into N-Space and some (such as the Ancient Lights, Great Old Ones and the Beast) had god-like powers. Thus, some of the most powerful beings in existence came from before the universe that was home to the Time Lords began. This made them older even than the Time Lords themselves. (NA: All-Consuming Fire, MA: Millennial Rites)

History
It was said that the Pre-Universe had a pink void, stars shaped like doughnuts and physical laws closer to what might have been understood as magic. (MA: Millennial Rites)

Knowledge of this universe was a closely guarded secret. The Doctor only knew about it as a result of his connection to the Matrix during his time as Lord President. (MA: Millennial Rites, DW: The Invasion of Time) Even then, the knowledge would have proven temporary: in his sixth and seventh incarnations, the Doctor was fully conversant with the details and specifics of this universe (NA: All-Consuming Fire, PDA: Synthespians™), whereas by his tenth it would almost be an article of religious faith to him that such a universe could not have existed. (DW:The Satan Pit)

Species
Amongst the races that claimed to be older than the universe itself were the Great Old Ones such as Azathoth, Yog-Sothoth and Shub-Niggurath. (NA: All-Consuming Fire, MA: Millennial Rites) On many planets, these beings became part of the legends and religions. The Great Old Ones were worshipped by the first intelligent species to evolve on Earth - the Silurians. (NA: All-Consuming Fire) It is unclear whether the Great Old Ones were a single species, or merely a term to describe any unimaginably powerful being that had escaped the Pre-Universe into ours.

The Beast claimed to have originated before the universe and to have been imprisoned by the Disciples of the Light, likely placing both in the Pre-Universe. The Beast was also said to have been the inspiration for all Satan-like figures in the mythology of countless alien races, including the Demons of human mythology, the ancient Kaled God of War and the Draconian God of Evil. (DW: The Impossible Planet / The Satan Pit)


 * In point of fact, the Beast claimed to have been imprisoned "Before Time," and debatably the Pre-Universe could still have been a part of "time".

Abaddon, who was the son of the Beast, probably also came from this period and was imprisoned around this time. (TW: End of Days) The Beast claimed both Abaddon and The Destroyer as alternate names - it is unclear whether the Destroyer was related to them in some way. (DW: The Impossible Planet)

The Ancient Lights were speculated by Luke Smith to have come from the Pre-Universe. (SJA: Secrets of the Stars)

Syriath, an entity who could raise the dead, was said by Jack Harkness to have been trapped within the Rift since before time. (BBCR: The House of the Dead)