Zygon Commander (The Day of the Doctor)
From Tardis Wiki, the free Doctor Who reference
A Zygon was a commander of a whole Zygon fleet trapped in the 11th 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.
