Skarasens are enormous cybernetically-enhanced reptilianoids used by the Zygons.
They are non-sapient aquatic predators which produce a form of lactic fluid that is consumed as nourishment by their masters. The adult Skarasen is a tough creature and can also be summoned by communicating devices to attack specific targets. It isn't known if this is a common practice by the Zygons, or one employed in times of limited resources; such being the case of the small group of Zygons which found themselves stranded on Earth following the destruction of their homeworld, and attempted to conquer humanity to inherit the new planet.
This particular Zygon group operated from their sunken spaceship at the bottom of Loch Ness, and allowed its Skarasen to swim from the loch to the North Sea using subterranean channels. It is safe to assume that the Skarasen in question became the source of the cryptozoological stories about a Loch Ness Monster. Although its Zygon masters ordered it to attack London from the Thames; the Earth defense force known as UNIT; helped by the Time Lord know as the Doctor; managed to destroy the aliens and free the Skarasen from their control, after which the beast retreaded back to the depths of Loch Ness.