Deep One (The Shadow Over Innsmouth)

The Deep Ones are a race of fish-like oceanic humanoids which are capable of interbreeding with Humans. A colony of them is known to exist somewhere in the South Pacific, near the French Polynesia, and they were brought to the USA's coast by Captain Obed Marsh in the mid-19th century, where they established another settlement called Y'hánthlei off the coastal city of Innsmouth, Massachusetts.

Biology
The Deep Ones are grotesque humanoids with features that resemble both fish and batrachians. They have dark grayish-green skin and white bellies. Their skin is mostly smooth, but the hide is covered in scales. Their faces are very fish-like in appearance and their eyes seem to lack eyelids. They have webbed hands and feet, and possess very noticeable gill slits on the sides of the neck.

Although generally aquatic, they're able to move on land with respectable speed and agility, although they aren't very gracious as they seem to randomly alternate between bipedal and quadrupedal gait. Their language is bestial but highly expressive, resembling all sorts of screams, screeches and barks.

Deep Ones never age and are considered biologically immortal; which constitutes one of the reasons why people would be tempted to interbreed with them.

Hybrids
Human hybrids grow up as relatively normal Human beings until adulthood, but soon start developing a progressively eerier appearance: large bulging eyes which appear to never blink, large hands and feet, slightly elliptical skull, poorly developed ears and chin, fleshy lips, rough and grayish skin, strange folds on the sides of the neck, early loss of hair and general absence of facial hair are among the most common symptoms. It's also said that animals such as cats, dogs and horses are instinctively hostile towards them.

As they age, their appearance deteriorates to the point that they can no longer pass as regular humans and are forced to hide themselves until the transformation is complete and they may escape into the ocean to live amongst the Deep Ones.

Culture
The Deep Ones live in vast underwater cities and appear to have a polytheistic religion, as they worship entities known as Mother Hydra and Father Dagon. The church known as the Esoteric Order of Dagon has become the main religion in Innsmouth since the time Obed Marsh made his pacts with the creatures. They also seem to worship Cthulhu.

The Deep One race is said to be much older than mankind and it appears that they were enemies of the Old Ones; meaning that they were probably allied with the Spawn of Cthulhu, assuming that the two coexisted in some period (but it's also possible that the Spawn were no longer around when the Deep Ones evolved).

They are familiar with Shoggoths and have access to live ones, meaning that they're either allied with them or able to domesticate or control them (the sapience level of modern Shoggoths is not clear, but they're probably not fully sapient).

Appearances

 * The Shadow Over Innsmouth, by H. P. Lovecraft (1931)  (First appearance)