The Rutan Host or simply Rutans are green amphibious creatures that have the ability to change their appearance at will. They come from the planet Ruta III and are lead by the Rutan Queen. They serve as the antagonists of the Doctor Who episode Horror Of Fang Rock.
The regime of the Rutan Host is a hive-minded empire which is ruled by the Rutan Queen. At one time, the Rutan Empire controlled all of the galaxy known to the Time Lords as "Muller's Spiral" (aka, the Milky Way Galaxy). The Rutan Empire has existed for at least 150 million years, as far back as the Millennium War. In which the Rutans were allied with other spacefaring civilizations (e.g. Time Lords, Osirans, Daemons, Kastarians, and Sontarans) in an alliance that deployed a 1000 fleets against the Constructors of Destiny in an epic war.
War with the Sontarans
The Rutans have been at war with a rival alien warrior race the Sontarans for thousands of years. Many other worlds such as Earth and Galifrey have been caught in the battle zone however neither side has ever been able to gain the advantage. Although the Sontarans have more advanced technology then the Rutans, the Rutan Host's shape shifting ability gives them a smaller upper hand.
"The Gunpowder Plot"
In the 13th century a Rutan ship crashed onto Earth after a collision with the TARDIS. Eventually after 400 years the British Parliament House was built on top of the Rutan ship which happened to contain a Rutan doomsday weapon as well. In the year 1605 the Rutans teamed up with Guy Fawkes to blow up the Parliament House so they can free their ship. However a squad of Sontarans try to steal their weapon and the two sides end up battling in the Parliament House. Eventually the Eleventh Doctor found out that the Rutans had two super weapons so he gave one to the Sontarans which led to a stale mate between both sides prompting them both to leave.
"Horror of Fang Rock"
During the 1900s the Rutans began to lose their war with the Sontarans, losing many planets and bases in the process. A Rutan scout was sent to the Fang Rock lighthouse on Earth to inpsoect that planet for it's usefulness in their war. The scout found that Earth made for a good base point for the Rutan Host in the war. However the scout was killed by the Fourth Doctor with the help of his companion Leela. Later the Doctor was able to destroy the Rutan mother ship over Earth.
"The Two Doctors"
During the late 20th century, Group Marshal Stike commented to the Sixth Doctor that the Rutans were on the verge of defeat at the hands of the Sontarans. The Sixth Doctor disagreed and advised him that his money is on the Rutans.
"The Poison Sky"
Later in 2009, the Sontarans attempted to convert the Earth into a world suitable for making Sontaran clones. The Tenth Doctor said that the tide had turned in the Rutan-Sontaran War, and the Rutan Empire was winning.
"The Taking of Chelsea 426"
In the 26th century on Chelsea 426, a city sized colony floating on the clouds of Saturn, the Rutans created a virus that allowed them to infect people and possess them but the Sontarans arrived and cured their victims of the virus.