Atlantis
Also known as Thuvaraiyam Pathi, the legendary lost city of Atlantis was rumored to have sunken deep beneath the sea after a terrible catastrophe. All records of and artifacts supposedly from Atlantis are vague and likely just rumors and fakes. No such island nation has ever been discovered.
Unknown to the surface dwellers, Atlantis has become an entire underwater nation, not just one city state. Cecaelia, deep ones, gillmen, grindylows, kelpie, merfolk, naiads, naga, nereids, nix, oceanids, rusalka, sahuagin, sea devils, selkie, undine, and many other creatures of the sea represent the civilized races and states of the federation of Atlantis. They have little care of what surface dwellers do except when they meddle with the oceans. Some Atlanteans travel to the surface as adventurers, explorers, or even pirates, but are reluctant to reveal the secrets of Atlantis to cany surface dwellers.
https://en.wikipedia.org/wiki/List_of_aquatic_humanoids
The amabie from Japanese folklore[6][a]
The ceasg in Scottish folklore has the upper body of a beautiful woman merging with the tail of a grilse (a young salmon)[7][8]
Finfolk from the folklore of Orkney
fuath, "malevolent humanoid water-spirits" from Scottish folklore[56][57]
Kappa from Japanese folklore
Melusine in European folklore[a]
Merrows from Irish folklore
Neck from Scandinavian folklore
Ningyo from Japanese folklore
Sirens were initially described as birdlike, but have become associated with mermaids in later folklore.
Umibōzu from Japanese folklore[a]
Undines in the writings of Paracelsus
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