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. She went down to the lower levels, where she ambushed the real Osgood and took her inhaler.
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. (TV: The Day of the Doctor)
The two Osgoods then created Operation Double, where a group of 20 million Zygons would be aloud 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 eachother in peace, and that no harm would come to anyone. However, one of the Osgoods was killed by Missy, and this lead to a revolt from a rouge group of Zygons who believed they had been betrayed. (TV: The Zygon Invasion)