The Zygons are a advanced bipedal species that developed space faring technology. They have shape-shifting abilities, allowing them to replicate the appearance of another being, but they must keep the subject alive in order to use its body print. Zygon technology is biological in nature: their ships and equipment are actually alive.
Their homeworld was destroyed some centuries ago when their sun exploded. A surviving craft made it to Earth, where it crashed in Loch Ness. They were also accompanied by an armoured cyborg creature called the Skarasen, the lactic fluid of which was necessary for them to feed. To allow a refugee fleet of Zygons to colonize the planet, they were planning during the 1970ies to conquer it by unleashing the Skarasen. The plan was foiled and they all perished in the attempt.