Lloigor
The Animus of Vortis, according to several accounts, was actually a Great Old One known as Lloigor. (PROSE: All-Consuming Fire, Millennial Rites, The Dark Path)
According to some accounts, "Lloigor" was a singular being, one of several who had survived the destruction of the universe before the Doctor's and gained godlike powers in the new reality; other Old Ones included such entities as Yog-Sothoth, otherwise known as the Great Intelligence, and Shub-Niggurath. (PROSE: All-Consuming Fire) These beings were clarified in other accounts to all have been members of an earlier race of Time Lords who ruled the old universe, (PROSE: Millennial Rites, Divided Loyalties) and one claimed that the names they were known by in the Doctor's time had been given to them by Rassilon, as opposed to being their original names. (PROSE: Divided Loyalties)
In another account, however, the Second Doctor described "the Lloigor" as a wider species to which the Animus belonged. Furthermore, he believed there might be many other Lloigor still living in their original universe, as opposed to the Animus being a survivor of a destroyed universe. (PROSE: The Dark Path)
There were several Lloigor originally, but the one that came through to our Universe used an awful lot of energy to get here, so I doubt that any others will be willing or able to expend enough strength to try anything so dramatic again.
Behind the scenes
August Derleth created the singular Lloigor. Colin Wilson created the plural Lloigor, a race of invisible energy beings that could manifest as dragons and "sea-devils". In Wilson's story, they are associated with the names of obscure Great Old Ones that H. P. Lovecraft named in stories he ghost-wrote. The protagonist of Wilson's story concludes that Derleth's Lloigor got its name from the race. Both Grant Morrison's Zenith and Alan Moore's The League of Extraordinary Gentlemen use the term Lloigor as a collective, plural term to to identify Great Old Ones created by Lovecraft or Derleth. The former is of particular interest to the Doctor Who universe due to making unlicensed use of the Order of the Black Sun from the DWM comic stories, renamed after a few issues as "the Cult of the Black Sun"; the Cult was then name-dropped in turn by Lawrence Miles in the Eighth Doctor novel Interference.
In his notes for the Sixth Doctor novel 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 identifed Lloigor as having been "that universe's equivalent of a Senior Watcher". Hinton viewed Lloigor as a single Great Old One, and he is referenced as such in Hinton's own Millennial Rites as well as in Andy Lane's All-Consuming Fire. In contrast, David A. McIntee's The Dark Path, also written by McIntee, implied a very different cosmology with multiple Lloigor of whom the Animus was only one.
