A Guide to Classic Who references in new Who including classic foes.
The Zygons are a race of shape changers who appeared in the classic story 'Terror of the Zygons" (Season 13).
In "Terror" the Doctor discovers a group of Zygons living near Loch Ness whose starship crashed centuries ago and had infiltrated the local community, taking on human form. Upon discovering that their homeworld had be destroyed in a stellar explosion, the group decide to take over the Earth.
They use the Skarasen (an aquatic creature resembling a marine dinosaur which provides them the milk the Zygons need to survive and has given rise to the legend of the Loch Ness Monster) to attack London.
The Time Crash blog was created to help New Who fans understand Classic Who references - and to know if something isn't a reference but a new idea. If there's a reference I missed or a subject that you feel needs more explaining, please comment.