Rutan

The Rutan are a technologically advanced gelatinous species.

Physiology
In their usual state, Rutans are large green spheres with tentacles, giving them a superficially jellyfish-like appearance. Their body is a few feet in diameter.

Rutans trained in 'special metamorphosis techniques' have the ability to take the form of any creature they desire. All Rutans can also generate lethal bioelectrical shocks, likely indicating the presence of electroplaques. Despite having no visible mouth, they can speak.

They seem to absorb electrical energy for sustenance.

History
The Rutans are at war with the Sontaran Empire, in a conflict which has been going on for fifty thousand years and never ends, despite the Sontaran Empire only lasting another ten thousand years (this might suggest that the Rutan host are winning the war, as the Sontarans would never surrender, even if they were almost wiped out).

The Doctor said that he would 'place [his] bets on the Rutans'.