The Sea Devil appeared in 1972 TV series called Doctor Who. The Sea Devils, once called the Aquatic Silurians, were amphibious Earth reptiles. Like their land-based cousins, the Silurians, they lived in prehistoric times in a scientifically-advanced civilization.
As their aquatic colonies awoke from suspended animation, these one-time rulers of the Earth attempted to reclaim their planet, leading to conflict with the new rulers: humankind.
The Sea Devils were related to the Silurians, though they were genetically divergent enough to create problems for the offspring of such mating.
Like the Silurians, variations existed among the Sea Devils with different subspecies and appearances. Sea Devils typically looked somewhat like humanoid turtles or plesiosaurs. They had beak-like mouths, large eyes, two fins on their head and long necks. They had three webbed digits on their hands and feet. Their skintones came in a variety of shades, including green, blue and orange. They were much tougher than humans, since they needed to survive the crushing depths of the ocean. Sea Devils communicated in a language like sonar, but they could speak human languages in a whispering, rasping voice. A Sea Devil's skin was slimy to the touch.
One group of Sea Devil warriors, which appeared at Sea Base 4, were notably distinguished by their silver skin and their lack of head fins.
Another subspecies of Sea Devil, suggested by the Twelfth Doctor to be a "warrior-class", were larger, faster, and stronger than those that he was familiar with. Their muscular bodies sported dark green skin, small red eyes, a pronounced jaw, and a large tail. They lived on the Raven Peninsula. They could communicate using a form of sound-based telepathy and could use this ability as a weapon by producing a deadly sonic scream.
There also existed a class of Sea Devils who were much smaller, about the size of an infant human, that could merge with a human host, creating a human-Sea Devil hybrid but with the Sea Devil's psyche dominating its host. They possessed the sound-based abilities of their warrior-class counterparts but also displayed potent psionic powers, specifically telekinesis. The Twelfth Doctor explained that this was a side effect of two minds merging into one.