Zygon Commander (The Day of the Doctor)

A Zygon was a commander of a whole Zygon fleet trapped in the 11th century. Using stasis cubes, the fleet planned to enter through to the 21st century. The Zygon first assumed the form of a horse that was used by the Tenth Doctor and Queen Elizabeth I. After being caught out, it's fled and assumed Elizabeth's form. Both Elizabeth and the Zygon fought to try and convince the Doctor that each of them was the real Elizabeth, before fleeing into a forest. Elizabeth later killed the Zygon, and then pretended to be the Zygon. (TV: The Day of the Doctor)