Rutan

The Rutan Host (or Rutans) were a race of amorphous green blobs who waged a millennia-long war with the Sontarans.

Biology
In their natural forms, the Rutans were shaped somewhat like jellyfish, with a green glowing body and many light-green tendrils. They were amphibious and could cling to sheer, vertical surfaces. They had considerable mobility out of the water, despite their shape. Rutans could see, even without visible eyes, but their vision was limited. (TV: Horror of Fang Rock) They could also speak by forming vocal apparatuses (PROSE: The Infinity Doctors) which gave them a harsh, tinny voice. (TV: Horror of Fang Rock) Rutans reproduced by binary fission, with two identical Rutans being produced. (PROSE: Shakedown)

Rutans were formidable fighters, even when they did not have weapons. Rutans could generate lethal shocks to defend themselves and seemed to be able to absorb electrical energy directly for sustenance. (TV: Horror of Fang Rock) They could also use this energy to produce a force field to absorb energy from weapons, though this required great effort. (PROSE: Shakedown) Though they preferred colder environments (TV: Horror of Fang Rock), Rutans could survive almost anywhere, including the vacuum of space. (PROSE: Shakedown)

Culture
The Seventh Doctor described the Rutans as "infinitely adaptable". (COMIC: Pureblood)

Rutans never referred to themselves as individuals, preferring the first person plurals "we", "us" and "our(s)". They also had no individual names. The Rutan hive mind was co-ordinated by a Queen who lived on Ruta III. Without her control the Host would fall apart. (PROSE: Shakedown)

Military tactics
Rutans were able to operate independently, but they did not see individuals as important and all individual desires were subsumed by the general desire to win the war with the "Sontaran rabble". (TV: Horror of Fang Rock)

Rutans used shapeshifting technology to adapt to alien environments and infiltrate alien cultures; they were consummate spies. A Rutan usually killed specific individuals and then impersonated them. (TV: Horror of Fang Rock, COMIC: Pureblood)

Technology
The Rutan Host had unknown types of technology, though they were apparently less advanced than the Sontarans. Rutan spaceships were crystalline in structure and looked like glowing balls of fire. These ships could control the weather to a degree. Individual Rutans could also shapeshift into other forms, but they needed to know the form very well, usually after careful study and dissection. (TV: Horror of Fang Rock)

Circa 1880 in England, Percival Ross managed to obtain Rutan healing salve and experimented with its ability to merge normally incompatible kinds of animal tissue, creating hybrids. (PROSE: Evolution)

Rutan biotechnology was sufficiently advanced to "uplift" a species to sentience. By the 26th century, they had done this at least once, on the planet Sentarion, where they were worshipped by the insectoid Sentarii as "the Shining Ones". (PROSE: Shakedown) They also created the Octopod as a biological weapon. (COMIC: Cyclops)

At some unknown point in the future, the Rutans were developing a new weapon that could destroy any planet by converting it from matter to anti-matter. (PROSE: The Infinity Doctors) At some point, they also used analysis engines. (PROSE: The Quantum Archangel)

History
The Rutan Host had been engaged in war with the Sontarans for thousands, if not millions, of years, though the two species had a brief alliance during the Millennium War. (PROSE: The Quantum Archangel)

At some point in their early history, the Rutans discovered a wormhole that linked Ruta III with Sentarion. The Rutans planned to use it as an escape route in emergencies and so modified the native species as guardians. The Rutans were considered to be "the Shining Ones" and the technology to open the wormhole was hidden in a temple and guarded by Harrubtii assassins. (PROSE: Shakedown)

In the dying days of the Fifty-first Star Fall Campaign, the Sontarans suffered great losses against the Rutans. (COMIC: The Instruments of War)

In 1199, a Rutan was inhabiting the Stockbridge Castle on Earth. (AUDIO: Castle of Fear)

In the 13th century, the Doctor's TARDIS clashed with a Rutan ship. The Rutans crashed and remained underground for 400 years, by which time the Houses of Parliament had been built on top of the crash site.

In 1605, the Rutans under the Houses of Parliament awoke from stasis and planned to take in the plot to blow up Parliament with Guy Fawkes to free their ship. At the same time, a platoon of Sontarans arrived to retrieve the Rutans' doomsday weapon which could wipe out either species. The two sides proceeded to battle in the Houses of Parliament for the weapon while the Eleventh Doctor, Amy and Rory attempted to stop them both. The Doctor, having discovered a second Rutan doomsday weapon, gave one to both sides, one retargeted to destroy the Rutans, without telling them which was which, thus creating a stalemate. The Rutans and the Sontarans both gave up and left Earth. (GAME: The Gunpowder Plot)

In 1609, Rutans participated in the Armageddon Convention, which convened in Laputa, a floating island in the sky near Venice. (PROSE: The Empire of Glass)

Though the Rutans originally controlled most of the Mutter's Spiral, by the 1900s, they were starting to lose the war and were forced to retreat to more fortified locations. During this time period, a Rutan scout landed on the coast of England and took over the Fang Rock lighthouse while attempting to assess the planet's usefulness as a base to fight the Sontarans. All their actions were to advance the war, such as scouting out new planets to use as bases. Both the scout and its mothership were destroyed by the Fourth Doctor. (TV: Horror of Fang Rock)

In 1918, a Rutan Octopod crashed onto Earth. It attacked the USS Cyclops, but was driven back by the Tenth Doctor and Heather McCrimmon. (COMIC: Cyclops)

In 1941, a Rutan spy disguised himself as a decorated Nazi veteran called Heinz Bruckner. As Bruckner, he sent artillery strikes in the Sahara Desert to pinpoint the location of the Sontaran "world engine" weapon the Warsong so he could use the Earth as a weapon against the Sontarans. After activating the weapon, the Twelfth Doctor stopped Bruckner's plans by using a Sontaran osmic projector to send Bruckner's trigger mechanism through time. He then blew up the weapon with his sonic screwdriver so that neither the Sontarans nor the Rutans could claim the weapon for themselves to use against the other side. (COMIC: The Instruments of War)

During the 21st century, a Rutan arrived on Earth and posed as a young swimmer called Emma who was a contestant in the globe games. The Rutan was forced to kill several humans to prevent them from revealing its true identity, until it was killed by one of a group of Sontarans that invaded BASE. (PROSE: The Sontaran Games)

During the 26th century, a Rutan known as Karne was working undercover as a Sontaran. When he learned that the Sontarans knew of the wormhole over Sentarion, he tried to return to the Rutans, but was wounded and stranded in space. Becoming known as "the Ripper", Karne started killing rich individuals, stealing their money and identity and using the cash to buy a flight to somewhere else. After being wounded on Megerra, Karne stowed away on the Tiger Moth. Commander Steg, who was tracking him down, was able to kill him on the Tiger Moth, but not before Karne was able to reproduce. The second Karne was able to get to Sentarion, where he intended to use the wormhole to escape to Ruta III. Unfortunately for him, the Doctor closed the wormhole to stop a Sontaran War-Wheel from reaching Ruta III. Karne was later killed by Steg. (PROSE: Shakedown)

During the Rassilon Era, the Rutan Host and the Sontaran Warburg convened on Gallifrey to make diplomatic overtures. (PROSE: The Infinity Doctors)

The Sixth Doctor stated (perhaps in an effort to spite his captors) that his money was "still on the Rutans" when it came to the Rutan-Sontaran War. (TV: The Two Doctors)

Undated events
Both Sontarans and Rutans were trapped in the Edifice. They continued to fight even there. (AUDIO: The Forever Trap)

A planet (defined as a "worthless rock" by the Sontarans) was an outpost of the Sontaran-Rutan war. Ended up on it, the Fifth Doctor, Adric, Tegan and Nyssa bumped into a troop of Rutans, that tried to imprison and kill them, in vain. (COMIC: Prisoners of Time)

Behind the scenes
The Third Doctor first mentioned the Rutans as the Sontarans' enemy during The Time Warrior, the first story to introduce the Sontarans.

The Fourth Doctor story Horror of Fang Rock established what the Rutans actually looked like and introduced their shape-changing ability. Interestingly, the Rutans have not, to date, actually appeared in a television story alongside their archenemies the Sontarans, though they were featured together in the Doctor Who Magazine comics story Pureblood, the BBV video Shakedown: Return of the Sontarans, the novel The Infinity Doctors, the New Series Adventures The Taking of Chelsea 426 and The Sontaran Games and the video game The Gunpowder Plot.