Hoothi creatures were masses of grey with bulbous heads and tendrils. They fed on blood and breathed gases poisonous to humans. Depending on how strong they were, a single Hoothi could only control a single slave.
The Hoothi were a group mind, usually travelling in fours that formed their own group minds within the greater Hoothi Consciousness. They could also access the memories and experiences of the beings they brought into the group mind, and it was possible for an absorbed being to regain their individuality and affect the rest of the group mind. A single Hoothi could still retain the millions of individuals that had been absorbed.
Their infestation spread through spores resembling white fibres that could enter various lifeforms, usually working better with more complex nervous systems. Infected beings could be identified by their retinal patterns, and the spores could be activated at any time, causing an infected creature to explode with grey fibres until fully absorbed. Creatures absorbed by the Hoothi included a Time Lord, humans, Heavenites, and a Terileptil. (PROSE: Love and War)
Hoothi travelled in silent gas dirigibles that were able to hide within hyperspace. These ships, constructed from animal bones and tissue, were usually crewed by four Hoothi holding psychic control over their slaves. The ships were undetectable because of the Hoothi's psychic powers. Bubbles could form inside a sphere to give visitors a breathable atmosphere. A staircase could descend from these ships to allow for boarding, and they were also able to enter Puterspace. (PROSE: Love and War, AUDIO: Love and War)
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. Hoothi spores had spread throughout time and space, even reaching 20th century Earth.
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. Before the Hoothi worlds could be time looped, the creatures had fled the 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)
Maren of the Sisterhood of Karn claimed that no ship could approach Karn without the Sisterhood knowing, saying that even the silent gas dirigibles of the Hoothi were felt in their bones. (TV: The Brain of Morbius)