In H. P. Lovecraft’s The Shadow over Innsmouth, the hybrid beings are the Deep Ones. They are intelligent, amphibious, fish-like humanoids who live in ancient underwater cities and interact with human communities along the coast.
The story’s central horror comes from the agreement between the Deep Ones and the people of Innsmouth. In exchange for abundant fish and strange gold jewelry, humans promise sacrifices and interbreeding. Their descendants may appear human when young, but many gradually develop the “Innsmouth look”: bulging eyes, shrinking ears, scaly skin, and gill-like folds around the neck.
The hybrids eventually undergo a transformation and are drawn to the sea, where they join the Deep Ones in Y’ha-nthlei, the undersea city beneath Devil Reef. The narrator discovers that his own ancestry is connected to this legacy, turning an external mystery into a personal revelation.
Cthulhu, Mi-Go, and the Elder Things are separate entities from Lovecraft’s wider Mythos. Cthulhu is an immense extraterrestrial entity, the Mi-Go are fungoid extraterrestrials, and the Elder Things are an ancient species described elsewhere.