Cecaelia
is the name often given to half-human, half-octopus creatures, though they may also be refereed to as octofolk.
Be very careful searching for images of this species because frankly a creature that is a beautiful human on the top half and gratuitous amounts of tentacles on the bottom half inspires a particular type of art from the internet.
Octopi, and cephalopods in general, have a number of abilities that make them interesting from a fiction or monster perspective.
Known to be highly intelligent
Willing to leave water
Associate with ocean floor, shallows, reefs and shorelines
Can change color
Masterful mimicry
Ink squirting
Carnivores
So you have a highly intelligent carnivore, master of disguise, that for some reason looks human on the top half.
Why on ‘Earth’ does this creature resemble a human?
Here is a simple octopus anatomy diagram for your consideration:
The octopus has no bones, but related species such as the squid and cuttlefish do have an internal shell, which serves a similar function. The Cecalia may have this sort of modified internal shell instead of a skeleton. This internal shell would allow the torso to be held upright, and additional internal shells may also allow the limb to move like a human.
However, this seems too simple to me. In the octopus and its tentacled friends, the mouth is located between the tentacles, and the eyes just above it. You are rearranging a lot of anatomy to move the eyes and mouth far away from their original position onto a humanoid torso, while presumably leaving the anus out of sight where it belongs.
What if the top half of the Cecalia is merely an imitation of a human torso?
I wouldn’t put it past a highly intelligent carnivore to mimic the upper half of an attractive human, especially if they have a culture that values shiny things you simply can’t mine under the ocean (gold and other loot)
It’s entirely plausible that the mouth of a Cecalia is still located between the tentacles like a regular octopus, with the real eyes just above the tentacles, but the ‘head’ of the octopus being reshaped to mimic a human. They may then attempt to lure humans into shallow waters to prey upon them.
The implications of such anatomy, where the humanoid torso is essentially an excellent mimicry, are:
The humanoid segment can resemble any humanoid, and can change
Different individual Cecalia could imitate the same humanoid at different times
A bold adventurer could ‘decapitate’ the humanoid torso and only seriously wound the octofolk.
Any voice the creature has is likely to emanate from its tentacle region.
They may leave the water for a short period of time but will always try to return there for a confrontation.
Octopi produce ink and have excellent vision. They are highly likely to be artistic.
But they have a completely different evolutionary pathway to humans and are unlikely to share the same goals, motivations to aesthetics.
If they have perfected the art of mimicking humans then they likely spend a significant portion of their time either preying on or scavenging from them.
Octopus eggs are cared for in batched of hundreds or thousands, and the newly hatched octopi are tiny when they emerge. Imagine all these tiny creatures innately trying to mimic humans but not quite getting it right. Nightmares, anyone?
So as far as an octopus mermaid goes, yes they probably would lure men onto the rocks, they probably would appear beautiful, they might not sing, but they are most definitely dangerous.
Fun fact, she was supposed to be WEARING that flannel shirt that’s two sizes too big for her, but I couldn’t get it to look right so I just tied it around her waist again. ¯\_(ツ)_/¯