Great Old Ones' universe

By some accounts, the Great Old Ones originated from the previous universe which had existed before N-Space, (PROSE: Millennial Rites, Synthespians™, Divided Loyalties) the "Space before this" which The Book of the Old Time described as having "Being before this" in the "Time before this". (PROSE: Christmas on a Rational Planet, The Quantum Archangel) This reality had different physical laws, more based in magic, which gave its survivors great power in other realities. (PROSE: Millennial Rites, Synthespians™, Divided Loyalties)

Some accounts indicate that the "Space before this" was actually N-Space in the pre-chronological time before the Time Lords imposed Rational history, (PROSE: Christmas on a Rational Planet, The Parliament of Rats) also known as the Dvapara Yuga, (PROSE: Dharmayuddha) Dark Time, Old Time, or Time of Chaos. (PROSE: Cat's Cradle: Time's Crucible, Christmas on a Rational Planet, et al.) This perspective viewed the 'previous universe' hypothesis as a misunderstanding of the truth arising from small-mindedness. (PROSE: Christmas on a Rational Planet)

Several accounts indicated that the Elder Gods originated in the Old Time, shortly after the Big Bang. (AUDIO: The Magic Mousetrap) Within this context, the Seventh Doctor nonetheless claimed that the Elder Gods originated from "the old universe". (AUDIO: Signs and Wonders)

The Warrior knew that the Old Ones in the Wilderness Beyond Time came from the universe before his own. (AUDIO: The Key To Key To Time)

Nature
The Time Lords had records of "the universe before theirs" in the Matrix, which the Sixth Doctor accessed in the Matrix during his time as Lord President. He later described it to Anne Travers:

"Before this universe was created, there was another one. A totally different universe, with alien physical laws. The heavens were green, and the stars looked like... giant doughnuts, to be brutally frank. Very, very different. And, as in this universe, there were people who discovered the deeper mysteries of time and space."

- Sixth Doctor

The belief this drove at was that the Great Old Ones, powerful beings who often seemed unbound by the laws of physics and whom the Doctor faced many times, were actually surviving members of the race which had been equivalent to the Time Lords in this previous universe. (PROSE: Millennial Rites)