4. Mermen
The Sea Devils (pictured) from the adventures of the Third Doctor (an arc adventurously titled "The Sea Devils") were amphibious reptiles based on Earth - very much like the famous Universal merman creation "Creature from the Black Lagoon" (1954). They were cousins of the land-based creatures known as Silurians and they lived in prehistoric times in a scientifically-advanced civilisation. Whenever their aquatic colonies awoke from their extensive periods of hibernation, these one-time rulers of the Earth would always attempt to reclaim what they saw as their planet - and this would usually leading to conflict with the new rulers, which are of course humans. Other swamp monster-like creatures in Doctor Who include the Marshmen of Alzarius - creatures who appeared in "Full Circle", an adventure of the Fourth Doctor. The Marshmen would emerge from the marshes every fifty years at a time known as Mistfall - an event that took place on the planet Alzarius approximately every half century - when another planet drew Alzarius away from its sun, which resulted in a cooling that generated a lot of mist.