Loch Ness was a famous lake in Scotland, home to the legendary Loch Ness Monster. It was connected to other Lochs via underwater tunnels. (PROSE: Sting of the Zygons)
The Skarasen was released into it by the Zygons and was identified as the Monster, (TV: Terror of the Zygons) as was the Borad when he arrived in the Loch (TV: Timelash) before being killed by the Time Lords to prevent interference with the events of the Zygons and Skarasen. (PROSE: The Taking of Planet 5)
However, the Seventh Doctor once stated that Loch Ness had been home to a thousand different monsters since the world was formed, from the first primeval weed-monster to a near-mythical spiny-headed sea-serpent. (PROSE: Christmas on a Rational Planet)