Transient Being

Great Old Ones such as Yog-Sothoth were described by the Sixth Doctor as "transient beings". (PROSE: Millennial Rites) Of these "Transient Beings", the Fifth Doctor said that they possessed the necessary time sensitivity for the operation of the Crystal Bucephalus, along with the Legions, Tharils, Eternals, and Chronovores. (PROSE: The Crystal Bucephalus)

Behind the scenes
In his notes for The Quantum Archangel, Craig Hinton elaborated on his concept of the wider cosmology of the Doctor Who universe, including the idea of the Great Old Ones as the survivors of an earlier race of Time Lords from the previous universe. Therein, he devoted a section to "the Transient Beings", defining them in contrast to the six Guardians of Time, the greatest of the Great Old Ones, who had been the High Council of the Old Time Lords. "The other Time Lords (those not attached to the Matrix but within the bubble that surrounded their Gallifrey) became the "evil from the dawn of time" types, also known as the Transient Beings – Yog-Sothoth, the Time Lord military strategist, became the Great Intelligence; Lloigor, a Senior Watcher, became the Animus; Shub-Niggurath, that universe’s equivalent of the head of the CIA, spawned the Nestene Consciousness… and so on. Importantly, none of them has any memory of the previous universe. All they want to do is conquer. Over time, they have built up a rivalry between themselves. Fenric and the like… But there are others."

- Craig Hinton

In Divided Loyalties, based in large part on this cosmology, Gary Russell employed a different terminology, mainly reserving the name of "Great Old Ones" to these "Sub-Guardians".