Hoothi

The Hoothi were fungoid creatures. The Seventh Doctor described them as the "the master strategists of the galaxy".

Biology and technology
The Hoothi worked as part of a group mind and spread their infestation through spores resembling fibres into various lifeforms. They could access the memories and experiences of the beings they brought into the group mind. They filled dirigible spheres with the gases they ingested. (PROSE: Love and War, AUDIO: Love and War)

History
While the Earth was still forming, Gallifrey did business with the Hoothi. It was unclear to the Seventh Doctor if they evolved or were artificially created. Some said that they evolved on a world where the climate changes wiped out all other life, described by the Doctor as growing up with death. They lived off decayed matter, infesting bodies and using them as pawns in a game.

The Time Lords tried negotiating with one world, but the Hoothi attacked them. The Time Lords sent an ambassador to the Hoothi worlds, but the Hoothi used him as a host. They attacked Gallifrey, but the Time Lords fought back and won with their advanced biochemistry. The Hoothi worlds were time looped by the Time Lords. The Hoothi then fled Mutter's Spiral.

At some point, they returned, using Heaven as a place to grow corpses, even manipulating the soil so that the bodies preserved better, waiting millions of years to begin a new attack against the Time Lords. They collected the Heavenites in cycles, eventually "recycling" all of the animal matter of the Heavenites. They eventually manipulated data so that Heaven became a graveyard for those that died in wars and controlled the Vacuum Church as pawns in their plan.

In 2570, the Hoothi awoke the dead. Through the manipulation of the Seventh Doctor, Jan Rydd used his pyrokinesis to set the Hoothi sphere that was over Heaven alight, killing three of the four Hoothi. Ace spoke to the part of the group mind containing her friend, Julian Milton, causing it to die. (PROSE: Love and War)

Other references
Upon seeing a toy Dalek in a child's toybox, Benny mused that there could have been a fluffy Hoothi in amongst the toys also. (PROSE: Sleepy)

Behind the scenes

 * A Sisterhood of Karn member in TV: The Brain of Morbius mentions the Hoothi. This was a mistake on the actress' part; it was scripted as Moothi. (DOC: Getting a Head)