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, with considerable mobility out of the water despite their shape. Rutans could see, even without visible eyes, but their vision was limited and monochromatic. They could also speak by forming vocal apparatus, giving them a harsh, tinny, male voice. Rutans reproduced by binary fission, with two identical Rutans being produced.
Rutans were considerable fighters, even when they did not have weapons. Rutans could generate lethal biolelectrical shocks to defend themselves and seem to be able to absorb electrical energy directly for sustenance. They could also use this energy to produce a force field to absorb energy from weapons, though this required great effort. Though they preferred colder environments, Rutans could survive almost anywhere, including the vacuum of space.
The Doctor described the Rutans as "infinitely adaptable".
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 influence the Host would fall apart.
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.
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.
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.
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".
At some unknown point in the future, the Rutans were developing a new weapon that was capable of destroying any planet by converting it from matter to anti-matter.
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.
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.
In 1880, a human found and experimented with Rutan biotechnology.
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.
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 and Karne was later killed by Steg.
During the Rassilon Era, the Rutan Host and the Sontaran Warburg convened on Gallifrey to make diplomatic overtures.
The Doctor has stated, perhaps jokingly, that his money is "on the Rutans" when it comes to the Rutan-Sontaran war.