The Rutan Host are 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.
War with the SontaransThe 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.
"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 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.