Deep Ones

The Deep Ones are a legendary non-human intelligent race which may or may not have existed in ancient times, during the Dawn Age.

Allegedly, they were a dark and vile race of half-men, half-fish creatures.

A few maesters speculate that the Deep Ones may have inhabited the Iron Islands before they were settled by the original ironborn (who were an offshoot of the First Men), and that they may have something to do with the mysterious black oily stone ruins which form the foundation of the Hightower in Oldtown - though these are fringe opinions without much support.

In the books
In the A Song of Ice and Fire novels, the Deep Ones are probably mythical and never actually existed...and yet, legends about them (or creatures similar to them) exist throughout all three continents in the known world.

According to the legends, the Deep Ones were a "queer, misshapen race of half men sired by creatures of the salt seas upon human women." These horrifying, half-human monsters lived in thrall to demonic gods and ancient beings from the deepest and darkest corners of the ocean.

Seastone Chair may have been raised by the Deep Ones before the ironborn. The theory is that the original ironborn drove off the Deep Ones from the isle, but similar to how the First Men on the mainland adopted the forest gods of the Children of the Forest, the ironborn's "Drowned God" religion was inspired by the beliefs of the Deep Ones, who similarly believe in all-powerful malignant deities who reign in the depths.

Stories claim the mazemakers were destroyed when foes rose up from the sea to destroy them, such as merlings, selkies (seal-men), or walrus-men.

Legend says that the Shield Islands at the mouth of the Mander were inhabited by merlings and selkies before the First Men arrived and drove them out.

"Merling" is the inclusive term for "Mermaids and Mermen".

Travelers to the Shivering Sea claim that the mermaids they spot have pale skin and black-scaled tails, far more dangerous than the pleasant mermaids of the southern oceans.

The folklore of Crackclaw Point, pine barrens whose inhabitants are largely poor holdout families descending from the First Men, have legends about "squishers": allegedly they appear human but have larger heads, and scales instead of hair. Allegedly, squishers have webbed fingers and toes, and their mouths have multiple rows of green, needle-like teeth. The legends say that the squishers were killed off when the First Men settled the peninsula, but some still frighten bad children by claiming that the squishers will come in the night to snatch them away, eating the boys but saving the girls to breed with. In ancient times, this loathsome race was said to be ruled by the "squisher king", with his grievous unhealed wounds.

Far to the east of Ib, off the north coast of the plains of the Jogos Nhai, are a large archipelago called the Thousand Isles; bleak, windswept rocks, some believe they are the only remnants of a kingdom which drowned in ancient times when the sea - or those who dwell within the depths of the sea - rose up to subsume it. The inhabitants of these isles are hostile to all outsiders, and with a forbidding appearance: hairless, with green-tinged skin, and filing their teeth into sharp points. The islanders worship statues and totems to bizarre fish-headed gods, which they set all along their coasts at low tide so they are partially submerged as the sea rises. If the hostile natives manage to capture foreign sailors who set foot on their isles, they kill them in dark rituals held in front of these idols, human sacrifices to their fish-gods. Despite living on an island chain, these natives live in great fear of the sea, and will absolutely refuse to set foot in the water or on a boat, even on pain of death. Apparently their fish-gods from the depths are not kind, and they do not revere them so much as hope to sate them by serving as their thralls, sacrificing others to those who dwell in the depths in hope of sating their hunger.

The Isle of Toads, part of the Basilisk Isles off the north coast of Sothoryos. There are ruins of some ancient civilization on the island, with large statues of foul gods set along the coasts. The largest of these is an ancient idol known as the Toad Stone, forty feet high and made of greasy black stone, crudely carved into the shape of a giant toad of malignant aspect. For the most part, the Basilisk Isles have no indigenous inhabitants, with centuries of slaver raids, colonization, and then crippling plagues totally depopulating them, after which they were resettled by the world's worst pirates. The one major exception to this, however, is the Isle of Toads: its modern inhabitants are theorized to be descendants of the long forgotten civilization that produced the ruins and the idols. The reason for this suspicion is the the locals don't resemble any other people, but are described as having an unpleasant, fish-like appearance to their faces, with webbed hands and feet.