The Rutans are a race of shapeshifting aliens that inhabit the universe of Doctor Who. They are best known as the ancient enemies of the Sontarans. They have the ability to change their physical appearance at will. Their natural appearance is a glistening, fluorescent green sphere approximately one meter in diameter, with a few tentacles thrown in for good measure. They are not a perfectly smooth sphere, as they have some small ridge patterns on their "skin".
They only make a single appearance in the television series. In The Horror of Fang Rock (1977, 4th Doctor) a Rutan vessel lands in the ocean nearby the Fang Rock lighthouse, just off the coast of England around 1900 AD. It has been determined that possession of Earth is of strategic importance in the war against the Sontarans. For a while the Rutan avoids detection by using the sea and darkness for cover, and by assuming the form of one of the lighthouse operators. Fortunately for the humans and the Earth, the Doctor and Leela are eventually able to thwart the Rutans by destroying them and their ship. The Doctor concludes that this will indicate to the Rutans that the Earth is too risky to invade. Personally, if I'm the Rutans and I've been fighting the Sontarans for thousands of years, the loss of one ship probably wouldn't stop me in my tracks. But that's just an editorial comment -- evidently the Doctor knew what he was talking about, because the Earth was not bothered by the Rutans again.
The Rutans appeared in the following episode of the television series: