Deep Ones

I think their predominant colour was a greyish-green, though they had white bellies. They were mostly shiny and slippery, but the ridges of their backs were scaly. Their forms vaguely suggested the anthropoid, while their heads were the heads of fish, with prodigious bulging eyes that never closed. At the sides of their necks were palpitating gills and their long paws were webbed. They hopped irregularly, sometimes on hind legs and sometimes on four… their croaking, baying voices… held all the dark shades of expression which their staring faces lacked.

H.P. Lovecraft, The Shadow over Innsmouth

Investigators first encountered Deep Ones and their human hybrids on the steamer SS Changsha on the way between China and Australia.

In Sydney, one of them was masquerading as human in bright daylight to approach the house where David Dodge lived and try to kill him.

In caverns below the fishing village on the outskirts of Melbourne a settlement of Deep Ones traded and breeded with human inhabitants of the village.

Pnakotic Manuscripts contain multiple notions of the aquatic race that, in the ancient epochs before humanity existed, had arrived from stars and warred against numerous other prehuman civilizations for the control over Earth.

Seven Cryptical Books of Hsan has chapters devoted to 'water spirits' and ways of dealing with them.

Necronomicon mentions the great priest Cthulhu, who lingers between sleep and death in his temple in R'lyeh.