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.

Biology
The Animus, like other Old Ones, had an octoid appearance, like an octopus crossed with a spider. 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. This fungus could replicate and repair itself, and whether it was the same material as or actually part of the Animus is unknown. While the Animus could apparently communicate with the Zarbi (though this was probably through their contact with gold), it spoke to other individuals through a helmet "hair-dryer" like device. This may have been a radio communicator, since it could speak directly to individuals who were with it in the Centre of the Carsinome. Whether the Animus had a sex or not is unknown, but it spoke with a feminine voice. (DW: The Web Planet)

When destroyed, the Animus still possessed a seed. Should the seed survive, it would receive the memory of the destroyed Animus and grow into a new creature. (MA: Twilight of the Gods) As it was part of the Lloigors, it may be that all Lloigors looked like and had the same biology as the Animus. (NA: All-Consuming Fire) The fact that the Animus could regrow itself, and whether or not the Carsinome was a biological extension of the Animus, could explain why the "Greater" Animus perished in the Last Great Time War. (DWAN: Doctor Who Annual 2006)

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. 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, and hinted that it could absorb the minds and knowledge of individuals. (DW: The Web Planet)

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. (NA: All-Consuming Fire/MA: The Dark Path)

At some point, it landed on Vortis and enslaved the peaceful Zarbi. 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, killing it. After that, the travellers left the natives of Vortis to rebuild their lives peacefully. (DW: The Web Planet)
 * It is possible the Animus had previous knowledge of Earth or encountered it before arriving on Vortis.

Sometime after, the Second Doctor, Jamie, and Victoria returned to Vortis and encountered a seed of the Animus which had survived. (MA: Twilight of the Gods)

Eventually, the Greater Animus perished in the Last Great Time War, and its Carsinome Walls fell into dust. (DWAN: Doctor Who Annual 2006)
 * As it was refered to as the "Greater" Animus, perhaps a weaker or smaller part of the Animus survived.

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.