The Skarasen were massive creatures, large enough to crush a human with one clawed foot. They were cybernetically altered to suit the Zygons' purposes and were vulnerable to no force short of a nuclear weapon. (TV: Terror of the Zygons) Their skins were highly durable and their skeletons were fused with an extremely tough alloy. (PROSE: Sting of the Zygons)
Originally from the Zygon homeworld, lots of Zygons transported embryo Skarasens to Earth in their ships, and the Skarasens were cybernetically enhanced to be weapons and so that the Zygons could see through their 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 that broadcasted an ultrasonic emulated mating call to the Skarasen. The Skarasen's presence in Loch Ness had helped to inspire the local legends of the Loch Ness Monster. (TV: Terror of the Zygons)
In 1894 in London, the Eighth Doctor encountered a group of Zygons and Skarasen babies. The Doctor (after nearly killing many of them by accident) helped the remaining Zygon move the baby Skarasens to an uninhabited planet to begin a colony. (PROSE: The Bodysnatchers)
In 1909, another group of Zygons came to Earth. Though they had brought two 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 Tenth Doctor and was allowed to escape. (PROSE: Sting of the Zygons)
In the mid-20th century, UNIT began investigating Loch Ness, so the Zygons sent out their Skarasen. Following the Fourth Doctor, who had a homing device in his pocket, the Skarasen travelled from Loch Ness to London via the Thames. The Doctor threw the homing device out for the Skarasen to fetch. Its subsequent fate remained unknown, though with the death of the Zygons on Earth it would have had no-one left to control it. The Doctor stated it went back to Loch Ness, the only home it had ever known. (TV: Terror of the Zygons) The event was covered up, with it being claimed the monster was an inflatable protest tool. (PROSE: The Secret Lives of Monsters)
An incarnation of the Master owned a Skarasen named Flipper and kept the creature in a tank in his TARDIS. The Master used to feed allies and enemies alike to the creature when they'd outlived their usefulness or because they were annoying him. In the end, the Master killed and barbecued the Skarasen. (PROSE: Lords and Masters)
Decades later, when Sarah Jane Smith and Rose Tyler vied to outdo each with the monsters they had seen while with the Doctor, Sarah ultimately managed to impress Rose by her statement that she had met the Loch Ness Monster. (TV: School Reunion)