Zygon Osgood

An unnamed Zygon was part of the group stuck on Earth. Their ship landed in 1562, but the group retreated into a statis chamber and stayed there until 2013, where this Zygon took Osgood's form after ambushing the real Osgood and took her inhaler. Osgood escaped, reclaiming her inhaler while doing so.

In the Under-Gallery, the Zygon attacked Clara Oswald, but fell back when the latter got hold of a vortex manipulator. Later, three of the Doctors erased the memories of both the humans and their Zygon counterparts, and peace was restored. When this Zygon suffered an asthma attack in Osgood form, the real Osgood quietly passed over her inhaler, allowing these two to secretly know which was human and which was Zygon. (TV: The Day of the Doctor)

The two Osgoods then created Operation Double, where a group of 20 million Zygons would be allowed to live on Earth and take on the form of the nearest person they could find. The treaty could only be maintained if both humans and Zygons agreed to live among each other in peace, and that no harm would come to anyone. They were given a device called the Osgood Box by the Doctor, to be used only in a dire emergency. However, one of the Osgoods was killed by Missy, (TV: Death in Heaven) and this lead to a revolt from a rogue group of Zygons who believed they had been betrayed. (TV: The Zygon Invasion)