In the Cthulhu Mythos, the Deep Ones are a fish-like and frog-like humanoid race whose main habitat is deep in the ocean (hence the "Deep" in "Deep Ones"). However, in spite of being mainly marine creatures, they will sometimes come up to the surface to make deals with humans: the get human sacrifices and various human knick-knacks, and the humans get gold and lots of fish in their waters, herded nearby by the Deep Ones.
Once such a deal is made, the Deep Ones will also convince the humans to start mating with them, the draw for the humans being that their children, born looking human, will become immortal when they fully turn into Deep Ones later in life; it's not certain what the Deep Ones get out of it. As a hybrid gets older they begin taking on more attributes of the Deep Ones: the eyes become bulging and unblinking, the head becomes narrow, the skin becomes scabrous (on the way to developing scales), the neck develops folds (on the way to becoming gills), the ears shrink, and they beginning going bald. When they become too obviously non-human, they are hidden away from outsiders, and eventual slip into the sea to go live in the Deep One cities on the ocean floor.