Animus

The Animus was one of several Great Old Ones known as Lloigors from the pre-universe. It was the only Lloigor to cross into this universe, afterwards landing on the planet Vortis. It could take over any living creature that was in contact with gold and manifested itself within an organic, self-healing palace called the Carsinome.

The First Doctor described the Animus as a creature of "almost unimaginable power" and "capable of great evil". (COMIC: Prisoners of Time)

Biology
The Animus resembled an octopus crossed with a spider or plant. It was able to create webbing (similarly to the Great Intelligence, another Old One) to ensnare beings and generated a fungus-like substance which grew into its fortress known as the Carsinome. While the Animus could apparently communicate with the Zarbi, it spoke to other individuals through a helmet "hair-dryer" like device. In the centre of the Carsinome, the Animus was able to communicate with individuals directly. The Animus spoke with a feminine voice. (TV: The Web Planet)

The Animus consumed organisms through direct mental absorption, unless they were unfit and only suitable for gross organic matter supply. The tentacles of the Animus spread out and divided like roots, generating webbing and absorbing matter wherever they touched. If they possessed a strong enough will, individuals who had been absorbed could possess a tentacle and separate itself from the Animus. Additionally, fission could also occur if the Animus was ever divided, resulting in two Animuses. Before dying, the Animus could create a seed of itself as a precaution that would receive the memory of the destroyed Animus and grow into a new creature. It would speak of its predecessor as if it were a new individual altogether. (PROSE: Twilight of the Gods) The Animus that the First Doctor met beneath Victorian London was able to regrow to its former self from a single tendril. It considered itself the same Animus that was nearly destroyed by the Isop-tope. (COMIC: Prisoners of Time)

Powers
The Animus was able to draw off power from the planet Vortis by having its Carsinome as the north magnetic pole. This attracted several other planets or moons towards Vortis (due to the Animus' use of isocryte). It was also able to siphon the water, drying up streams and leaving the planet barren with a thin, radified atmosphere. It could control any creature that was in contact with gold and seemed able to telepathically communicate with its Zarbi slaves. The Animus could physically generate a light that seemed to either hypnotise and/or weaken individuals. (TV: The Web Planet)

It absorbed isocryte and could master its anti-gravitic properties to create force fields, gravity beams, and draw other celestial bodies to it, eventually mastering flight. It could also mentally control people directly, without gold, but this used a lot of energy. It could also create web beings that had four arms and were incredibly resilient. It also created a plant that could recreate web beings in the form of a dead organism as a copy that would be passable upon quick inspection. (PROSE: Twilight of the Gods)

History
Like the other Lloigors and Great Old Ones, the Animus originated in the pre-universe. It was the only Lloigor to have enough energy to cross into N-Space and as a result of the physical laws, became a god. (PROSE: All-Consuming Fire, PROSE: The Dark Path)

At some point, it landed on Vortis and enslaved the peaceful Zarbi, receiving its name from the Menoptera. The Animus was introduced to Vortis by Twel in an effort to interfere with Bris's and Ilex's experiment. (PROSE: Twilight of the Gods)

The First Doctor, Ian, Barbara, and Vicki arrived and helped the Menoptera to attack the Animus. It seems its intention was to grow its Carsinome across the entire surface of Vortis, thereby taking over. However, upon encountering the Doctor, it became interested in reaching beyond the Isop Galaxy to "pluck from Earth its myriad techniques and take from Man his mastery of space." Barbara was able to use the Menoptera's secret weapon, the Isop-tope, to rapidly decay the Animus' cells. (TV: The Web Planet) The Isop-tope weapon shrivelled the Animus to "the slightest tendril". This was enough for the Animus to regrow. (COMIC: Prisoners of Time) After that, the travellers left the natives of Vortis to rebuild their lives peacefully. (TV: The Web Planet)

The Animus, having regrown its Carsinome, was transported to the London Underground in 1868 with an army of Zarbi by a cloaked force so it could conquer Earth. As the army came through, the First Doctor and Thomas Huxley distracted it, as Ian drove a train into the Animus' Carsinome, killing it. (COMIC: Prisoners of Time)

Sometime after, the Second Doctor, Jamie McCrimmon, and Victoria Waterfield returned to Vortis and encountered the seed of the Animus. This Animus had a voice that was neither male nor female and viewed its predecessor's plan as full of mistakes. It manipulated the more radical leaders of both sides of the Rhumon in their civil war and had them mine isocryte, bringing the mineral to an island where the Animus awaited. Flooding an acid pool with the ocean, the Animus absorbed the isocryte and began spreading, planning to consume half of the world in a day. In an effort to damage it, a bomb actually divided the Animus into two beings. Oryl of the Gods of Light fought the Animus before being damaged and retreating.

Utilising holographic technology that would keep them from feeling the Animus but allowing them to physically attack it, the Doctor created giant versions of Jamie, the two Rhumon leaders, and Kretus the Menoptera. They attacked both Animus individuals and ripped them apart, crushing their heart/brains and killing the spawn of the Animus. (PROSE: Twilight of the Gods)

The Seventh Doctor, Lysandra Aristedes and Sally Morgan encountered the Animus during their travels in the black TARDIS. (AUDIO: Black and White)

Behind the scenes

 * Lloigor (and its twin Zhar) was created by H. P. Lovecraft's contemporary, August Derleth, who also made Hastur and the legendary Windigo into Great Old Ones. The singular reference in Doctor Who to a plural Lloigor comes from a later Colin Wilson story which suggests that the fictional Lloigor got its name from a very real group of beings.
 * According to to REF: Doctor Who Annual 2006, a non-narrative source, the "Greater" Animus perished as a result of the Last Great Time War, its Carsenome Walls crumbling into dust.