I remember having read a few months ago in a bestiary preview something about a creature taken from the Ctulhu mythos. It was a unique named monster with a very high CR (>20).
It looked humanoid with a pale tentacly-face.
I crawled the Internet yesterday but I couldn't find it. Can someone help me? I am more precisely looking for the illustration of the creature but I think I can find it from the name.
Here is a small list of all creatures from cthulhu mythos published on bestiary books:
Bestiary - Ghoul, Shoggoth, Skum (Deep One)
Bestiary 2 - Denizen of Leng, Gug, Hound of Tindalos, Leng Spider, Serpentfolk, Shantak
Bestiary 3 - Moon-Beast, Voonith, Yithian, Zoog
Bestiary 4 - Bhole (CR 17) - Colour Out of Space (CR 10) - Demon Lord, Dagon (CR 28) - Elder Thing (CR 5) - Flying Polyp (CR 14) - Great Old One, Bokrug (CR 27) - Great Old One, Cthulhu (CR 30) - Great Old One, Hastur (CR 29) - Mi-Go (CR 6) - Nightgaunt (CR 4) - Ratling (CR 2) - Spawn of Yog-Sothoth (CR 10) - Star Spawn of Cthulhu (CR 20)
Bestiary 5 - Deep One (CR 1) - Deep One Hybrid (template) - Elder Deep One (CR 14) - Leng Ghoul (CR 10)
There are also creatures that are unique from paizo books and have a mythos appearance, like the Venedaemon, but that are not part of the cthulhu mythos.
You could also be looking at the preview image of the Great Old One Tawil at'Umr, that was previewed for bestiary 6 few months ago.