The Skarasens are massive creatures, large enough to crush a human with one clawed foot, which are cybernetically altered to suit the Zygons' purposes. It had vulnerability to no force short of a nuclear weapon. Their skin is highly durable and their skeleton is fused with an extremely tough alloy.
As well as being used as defence, Zygons feed on the milk of the Skarasen. This milk, a green ooze, is vitally needed by the Zygons to survive.
Originally from the Zygon homeworld, many groups of Zygons transported the Skarasen to Earth in their ships with them as an embryo and cybernetically enhanced it as a weapon and see through its eyes via monitor screens.
The group of Zygons who came to Earth in the 12th century brought a Skarasen with them. The Skarasen attacked any persons or objects to which the Zygons attached a transceiver which broadcast an ultrasonic mating call to the Skarasen. The Skarasen's presence in Loch Ness had helped to inspire the local legends of the Loch Ness Monster. Following the Doctor, who had the homing device in his pocket, the Skarasen travelled from Loch Ness to London via the Thames to follow him. The Doctor threw the homing device after for the Skarasen to fetch. Its subsequent fate remains unknown, though with the death of the Zygons on Earth it would have no one left to control it.
In 1894 in London The Doctor encountered a group of Zygons and Skarasen babies in London, the Doctor (after nearly killing many of them, by accident) helps the remaining Zygon move the baby Skarasens to an uninhabited planet to begin a colony.
In 1909, another group of Zygons came to Earth. Though they had brought 2 Skarasen, their minds were damaged by the solar radiation brought on during the destruction of Zygor and one was killed during the attempt to repair the damage. The other one was cured by the Doctor and was allowed to escape.