The Lake Monsters were a species of flat, tentacled, predatory mutants with glowing eyes who existed on Skaro.
One Lake Monster lay deep in the Lake of Mutations and killed Elyon by pulling him beneath the water. (TV: "The Expedition")
Fifty cycles after the Thal-Dalek battle, a Lake Monster observed the Doctor's TARDIS returning to Skaro. (WC: A return to Skaro for the First Doctor...)