Umibouzu

Umibouzu (海坊主, lit. Sea Priest) is a race of water youkai that appear throughout the GeGeGe no Kitarou franchise.
Umibouzu is a large youkai with black skin, it is unknown what most of his body looks as the rest is submerged underwater, though he does at least have a left arm. His face is rather simple, having a pair of large eyes with white sclerae, black irises, and white pupils.
Some of Umibouzu were originally humans that transformed into mermen or youkai.
Within the 2018 anime, Umibouzu appears with black, rubbery skin and lacking hair, he is quite big as most of his body is submerged underwater, though he has shown that he has a left arm. He has a rather simple face, having large eyes with light blue sclerae, black irises, and white pupils, his mouth resembles that of a squid's, being cylindrical and grey at the lips.

(Source: GeGeGe no Kitarou)