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. (PROSE: All-Consuming Fire, Millennial Rites, TV: The Impossible Planet)
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. (PROSE: Millennial Rites, TV: 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 (PROSE: All-Consuming Fire, Synthespians™), whereas by his tenth it would be taken with near absolute certainty that such a universe could not have existed. (TV: The Satan Pit)
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. (PROSE: All-Consuming Fire, 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. (PROSE: 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. (TV: The Impossible Planet / The Satan Pit)
The Beast actually said that he was chained 'Before time'... 'Before time, and light, and space, and matter'... 'Before the cataclysm'. Later, the doctor refers to the beast as having said 'Before the universe', which is the Doctor's fault for mis-hearing what the beast actually said. Several of the species on this page we know to have come from the universe before this one. Unlike those, one interpretation of 'before time' is that the beast was imprisoned even before the cycle of Big bang > Universe > Collapse > Big Bang. There would have been no time, no light, no space, no matter. The cataclysm, for something existing back then, might have been the start of this cycle, the very very first big bang, and not just the most recent one.