Rutan

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

Physical characteristics
Rutan host were amphibious and native to the cold planet Ruta III. They had trouble dealing with warm environments. (DW: Horror of Fang Rock)

In their natural forms, the Rutans (singular Rutan) 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, with considerable mobility out of the water despite their shape. Rutans could also generate lethal biolelectrical shocks able to kill Humans (and probably Sontarans), and seem to be able to absorb electrical energy directly for sustenance. (DW: Horror of Fang Rock) They reproduced by a process similar to binary fission, with each daughter Rutan carrying the memories of the parent. (Source?)

Rutans could see, even without visible eyes, but their vision was limited and monochromatic. (DW: Horror of Fang Rock) They could also speak by forming vocal apparatus (PDA: The Infinity Doctors), giving them a harsh, tinny, male voice. (DW: Horror of Fang Rock).

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

Rutans never referred to themselves as individuals, preferring the first person plurals "we", "us" and "our(s)". They also had no individual names. (DW: Horror of Fang Rock onwards)

The Rutan hive mind was co-ordinated by a Queen who lived on Ruta III. Without her influence the Host would fall apart. (NA: Shakedown)

Military tactics
Rutans were able to operate independently (and like the Sontarans, often dispatched scout units consisting of a single soldier), 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" (as they often unflatteringly call them).

They often used shapeshifting technology to adapt to alien environments and infiltrate alien cultures; Rutans were consummate spies. A Rutan usually killed specific individuals and then impersonated them. (DW: Horror of Fang Rock, DWM: 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 had the appearance of 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 this form very well, usually after careful study and dissection. This skill was discovered and implemented in the early 1900s. (DW: Horror of Fang Rock)

Circa 1880 in England, Percival Ross managed to obtain Rutan healing salve and experimented with its ability to merge together normally incompatible kinds of animal tissue, creating hybrids. (MA: 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 are worshipped by the insectoid Sentarii as "the Shining Ones". (NA: Shakedown)

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. (PDA: The Quantum Archangel)

In 1880, a human found and experimented with Rutan biotechnology. (MA: Evolution)

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 off the Sontarans. All their actions have been to advance the war, such as scouting out new planets to use as bases. (DW: Horror of Fang Rock)

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


 * We do not know when the Rassilon Era took place, relative to the history of Earth or other worlds. Also, we do not know when this conference took place relative to events in the known history of the Time Lords, such as the Sontarans' brief invasion of Gallifrey.

The Doctor has stated, perhaps jokingly, that his money is "on the Rutans" when it comes to the Rutan-Sontaran war. (DW: The Two Doctors)

Behind the Scenes
The Doctor first mentioned the Rutans as the Sontarans' enemy during The Time Warrior, the first story to introduce the Sontarans. Horror of Fang Rock established what the Rutans actually looked like and introduced their shape-changing ability.