Skarasen

The Skarasens are large reptilian creatures controlled by the Zygons. The Skarasens are massive creatures, able to crush a human with one foot, which are cybernetically altered to suit the Zygons' purposes. The Skarasen was originally from the Zygon homeworld, but one was brought to Earth as an embryo and was cybernetically enhanced to give it more strength and a powerful bite. Nothing short of a nuclear weapon could stop it. The Zygons fed off its lactic fluids and also controlled it with special ultrasonic signals.

A Skarasen was encountered by the Fourth Doctor, Sarah Jane Smith, and Harry Sullivan in Terror of the Zygons. In the story, a Skarasen was living near Loch Ness, Scotland and was mistaken by the locals for the mythical Loch Ness Monster. The Zygons were attempting to take over Earth using the Skarasen's strength and power.