Out of the blue, one of my sons asked me today what Cthulhu's "stats" were. My immediate thought was to look them up in the original Advanced Dungeons & Dragons Deities & Demigods. Having shown him the picture of Cthulhu there, I paged through the rest of the section, looking at the other pictures, and I was struck by something I had not noticed before.
The picture of the alien race from At the Mountains of Madness (called an "Old One" in the Lovecraft's original story, usually later known as an "Elder Thing," and here a "Primordial One"), although I had seen it before, struck a chord of similarity in me that I had not recognized before.*
Nowadays, you can find quite a few images of this alien race, showing a fair amount of variety in how the pentagonally-symmetric bodies are depicted. However, I noticed a surprising similarity between the Deities & Demigods illustration (which you can see is dated '80) and another one that had been published the year before, in Barlowe's Guide to Extraterrestrials. Barlowe's illustration might have been essentially the only one in circulation in 1980, and it probably influenced a lot of later pictures by showing only two visible wings.
However, there seems to me that there are some more specific similarities between the two pictures. The overall proportions—of all the body parts except for the head—are quite close. And while the head on the AD&D monster is somewhat larger, the fringe of eyes, as well as open and closed mouths, look very close to what Wayne Barlowe's closeups on the facing page show.
(I apologize for the poor quality of the picture, but it's all I could find online, and I can't find my own copy of Barlowe's Guide to Extraterrestrials to scan in a better image.) The greatest similarity though, it seems to me, is in the way the large lower limbs are stretched out on the ground in both pictures; they don't run in the same directions, but somehow the overall perspective of the way they are posed seems very, very similar.
So, did Erol Otis, who created the AD&D illustration, base it on Barlowe's (at the time) recent work? Or are the similarities I see just coincidental?
I recognize that this question sounds like something that it might be difficult to do more than speculate about. However, Erol Otis is active on the Internet and has, in the past, given some very interesting interviews about the ups and downs of his time work at TSR. So it's possible that he might have discussed or posted about this at some point in the past.
*I also noticed that they were labeled as Lawful evil (and the description below bears out an evil alignment), which hardly seems fair: Radiates, vegetables, monstrosities, star spawn — whatever they had been, they were men!