Hive mind

A hive mind, (TV: Asylum of the Daleks, Death in Heaven, WC: Pond Life) group mind, gestalt or gestalt consciousness, or mass mind (COMIC: Culture Shock!) was a collection of separate, distinct organisms with a shared consciousness. (TV: Planet of the Ood)

Richard Harries speculated that all beings had a collective memory that could be accessed, and experimented on rats to test this hypothesis. (PROSE: The Banquo Legacy)

Overview
The general idea of a hive mind was that the same mind existed in separate beings, or that each being's mind was linked with the others, allowing all organisms to know what was happening to any one organism of the hive.

The Wirrn could communicate with each other enough to allow them to move as packs. (TV: The Ark in Space) Similarly the Toclafane could communicate with each other remotely to perform tasks in groups (TV: The Sound of Drums) and had a shared memory. (TV: Last of the Time Lords)

The Seventh Doctor bumped into a sentient cell culture whose mass mind was able to send him a telepathic distress call. (COMIC: Culture Shock!)

Raxacoricofallapatorians of the Slitheen family were linked so that when one was electrocuted, they all were, and they would know when one of their family were killed. (TV: World War Three)

The Yulians of Yula possessed a hive mind, which they called the Communal Will, to which each individual fed their psyche and in turn fed from the psyches of the others. Krem-ling planned to seize control of the Communal Will, which would have enabled him to feed off of entire worlds at a time instead of individuals. After Krem-ling was destroyed by his servant Marsalla, she became part of the Communal Will as well. (PROSE: The Vampires of Crellium)

The Ood communicated telepathically with each other in the form of singing. (TV: Planet of the Ood)

Individuality
In some instances, group minds meant the creatures were without individuality, with each individual of the hive referring to itself as "we" or "us" rather than "I", showing that it spoke for the entire hive mind. (TV: The Impossible Planet)

The Ood were psychically linked through a hive mind (WC: Pond Life) but still retained their individualism. (TV: Planet of the Ood)

The Nestenes made up a Universal Mind, with Swarm Leaders being able to directly control an Auton. Despite all Nestenes being linked, Channing seemed surprised to learn the Replica of Scobie was destroyed, implying he was unaware. (TV: Spearhead from Space)

Similarly, although the Rutans were described as all being one, the Rutan agent Karne attempted to get a message to his homeworld warning about the Sontarans, rather than the entire Host automatically knowing. (PROSE: Shakedown)

Leaders
Some hive minds even had leaders that directed them, most often referred to as a “Queen.”

The Amaryll were led by a Controller on Alvega. When a Dalek killed it at the planet’s core, the entire planet died with it. (COMIC: The Amaryll Challenge)

During the Second Nestene invasion of Earth, the Auton Leader reported to the Nestene High Command. (PROSE: Doctor Who and the Terror of th Autons)

The Rutans were so tied to their Queen Mother that were she killed, the Rutans would either go mad or die instantly. (PROSE: Shakedown)

The Wirrn Queen (PROSE: Placebo Effect) as well as other leaders could give orders through the hive mind. (TV: The Ark in Space)

The Tractators, though usually unintelligent, required the Gravis to direct them, similar to a queen bee and her swarm. (TV: Frontios) The Xaranti were similarly less intelligent unless directed by a Queen. (PROSE: Deep Blue) The Skithra were another scorpion-like race of scavengers led by a Queen. (TV: Nikola Tesla's Night of Terror)

The Ood were a gestalt kept together peacefully by all being connected to a third brain, the Ood Brain, as well as having their forebrain and hind brain. (TV: Planet of the Ood)

Gestalts
Gestalts were creatures that not only shared a collective mind but together made up a single entity. Axos was a "composite creature" whose individual units were different but still linked psychically. (TV: The Claws of Axos) The Fendahl was made up of twelve Fendahleen and the Core, with a High Priestess seeming to have control. (TV: Image of the Fendahl)

The Squall were an extreme example of a gestalt consciousness; the sum intelligence of the collective Squall consciousness was distributed in small quantities. A lone Squall was barely smarter than an animal, but a whole pack was capable of speaking in full sentences and near-genius intelligence. (PROSE: Paradox Lost)

The Psionovores had an undefined natural energy existence, but could coalesce into a gestalt physical form, given enough will. (AUDIO: Minuet in Hell)

An intelligence on a planet visited by the Fifth Doctor tried to lure travellers to join its gestalt, almost succeeding with Adric. (PROSE: Hearts of Stone)

The Vashta Nerada were a gestalt of countless microscopic creatures that, due to their number, could devour anything in seconds. Each being part of a hive mind, they could work together to "walk" in a spacesuit. (TV: Silence in the Library / Forest of the Dead)

Artificial hive minds
Advanced networks similar to hive minds could be created technologically.

Skagra
Skagra attempted to use his sphere to put the same mind into every living thing in the universe, creating a universal mind, before he was defeated by the Fourth Doctor. (TV: Shada)

Daleks
Although the Daleks did not have a hive mind in the true sense of the word, (TV: Asylum of the Daleks) both the Twelfth Doctor, (TV: Twice Upon a Time) and Oswin Oswald compared the pathweb to a hive mind. They shared information through the web, and a single Dalek could delete information and all Daleks would forget. (TV: Asylum of the Daleks) The Sixth Doctor described it “artificial telepathy." (AUDIO: The Curse of Davros)

Cybermen
Various Cyber-subspecies operated via the usage of a hive mind or neural network, such as the Cybermen of the Mondasian colony ship, (TV: World Enough and Time) and those created by. (TV: Death in Heaven) The Central Committee of Mondas was created by connecting twenty of the greatest Mondasian minds. (AUDIO: Spare Parts)

The faction of Cybermen who had invaded the Federation universe were connected through the cyber-web. (COMIC: Assimilation²) After the Cyber-Wars, the Cyberiad was the general collection and connection of all Cyberman minds. Millions of Cybermen could be simultaneously controlled by a Cyber-Planner through it. (TV: Nightmare in Silver) The Cybermen of the Cyber-Empire had their own neural network, but it was possible to disconnect their Cyber-Warriors from it, though the hive was capable of detecting this. (TV: The Timeless Children)

Other hive minds
In the Federation universe, the Borg operated a hive mind comparable to the Cybermen. (COMIC: Assimilation²)

The Jixen had a collective consciousness. (TV: Hound of the Korven)

The Stenza had a hive mind, containing all their collective knowledge, which could be accessed even by an individual who had been separated from the rest of their kind. (TV: The Battle of Ranskoor Av Kolos)

In the far future of the 57th Segment of Time, the Darkness had evolved into a hive mind from Earth insects. (PROSE: The Well-Mannered War)

At the end of the universe, the last humans on Ember attempted to transform the people of 21st century Earth into a group mind they called "Singularity" through the Somnus Foundation, but were defeated by the Fifth Doctor and Turlough. (AUDIO: Singularity)

Other known hive minds included the Chronomites, (GAME: TARDIS) the Gappa, (PROSE: Snowglobe 7) and the Skorpius Flies. (TV: Dreamland)

Possible hive minds
Because of their abilities and similarities to known hive minds, both the Vespiforms (TV: The Unicorn and the Wasp) and the Hervoken may have been hive minds as well. (PROSE: Forever Autumn

Torchwood theorised that Weevils had a low-level telepathic ability to share emotions. (TV: Combat)