Pre-universe

The Pre-Universe was the Universe, or perhaps many Universes, which came before this one. After the destruction of the Pre-Universe, some species found their way into this Universe, and some (such as the Great Old Ones, and The Beast) had god-like powers. Thus, some of the most powerful beings in existence came from before the Universe itself was created, making them older even than the Time Lords. (NA: All-Consuming Fire, MA: Millennial Rites) Some beings, such as The Beast, claimed to come from before time itself, which may indicate a time before any Universes at all yet existed.

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

Knowledge of this universe was a closely guarded secret, and was only in the Doctor's possession 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 ultimately prove temporary; In his Sixth and seventh life, 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 Impossible Planet)

Any being able to survive the end of a Universe itself, must be unimaginably powerful, knolwedgable, or both, and thus, beings that claimed to originate from before the current Universe were amongst the most dangerous in existence. Indeed, out of the few beings more powerful than the Time Lords, many claimed to be from before the Universe, or before Time.

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. 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. It is unclear whether the Great Old Ones were a single species, or merely a term to describe any unimaginably powerful being that was able to escape the Pre-Universe into ours. If the latter is true, then the Beast may be an Old One too, and not all Old Ones are necessarily equal in age or power.

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. (DW: The Impossible Planet / The Satan Pit) 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.


 * Though in point of fact the Beast claimed to have been imprisoned "Before Time," and debatably the Pre-Universe could still be a part of "time". He could mean before the time of the current universe.

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

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