This concept is my version of what Cthulhu would look like. I know there are important differences between this one and the creature that Lovecraft described in his books, but everything has a reason! I'll try to explain the decisions that I made to complete my design.
It all started with a quick and dirty sketch that I made on my sketchbook, using a ball pen. I liked the idea that Cthulhu is a massive larger than life creature, so enormous that it cannot be understood only as a living being, but almost as a landscape. So I added a lot of details, like scales, holes and protuberances, so it would feel big as a mountain, with his own "caves”, depressions and tentacle forests... His wings are ridiculously little in comparison with the rest of the body. Maybe a Great Old One does not need such tricks as wings in order to fly, or maybe we are looking at a still-not-fully-developed larva ...
I know what you are thinking. This Cthulhu does not have a neck! I thought that, for a 1km tall being, a neck is a very fragile structure that would easily collapse, specially considering the amount of tentacles that hang from his head. I considered the body as a big fat mass of flesh, and a little head reinforces the size of the rest of the body.
How does Cthulhu breathe? Does he inhale and exhale great amounts of air and hold them in his lungs? Does he have lungs at all? Maybe those big holes on its body could explain part of the mystery, and also gives Cthulhu an alien look (it reminds me of Monster Bu from Dragon Ball Z!).
At first, I thought that Cthulhu should have mollusk wings, similar to the fins that squids and cuttlefishes have, but I couldn’t find the way to make it work. Still, those wings would not let him leave the ground, but at least they give him a more menacing look.
Why exactly would Cthulhu need hands? He’s so big that he doesn’t need to manipulate objects, and if he did, I think that some tentacles would be far more useful than a pair of giant claws. I depicted Cthulhu’s forearms as an amalgamation of claws and octopus beaks, that he would use to catch and destroy. In the center of what we could call “palm”, a mouth-like hole opens, from where sensitive tentacles can emerge to detect and manipulate little objects (let’s say, objects the size of a bus, for example).
The beard of tentacles is also interesting. I think of Cthulhu as a mollusk based lifeform, so almost every aspect of it has to be related to that kind of animals. I think that it wouldn’t make much sense if Cthulhu lacked tentacles in the most part of the body but the head, so I extended the area covered by them to the chest and part of the belly. The mass of tentacles, and the incapability of knowing where the mouth begins or ends, suggests a big overture in the body that spills the tentacles as scattered guts.
His legs are enormous pillars that have to hold a lot of weight. To compensate the pressure of carrying his body, several tentacles grow from his back, pushing the weight forward.
That's about it! I hope you like it!