In "The Slithering Shadow," Conan fights Thog, the monster-god of the city of Xuthal. Both the thing's name and description are suggestive of Clark Ashton Smith's creation Tsathoggua.
Something huge and bulky grew up out of the void. She saw a great misshapen head emerging into the light. At least she took it for a head, though it was not the member of any sane or normal creature. She saw a great toadlike face, the features of which were as dim and unstable as those of a specter seen in a mirror of nightmare. Great pools of light that might have been eyes blinked at her, and she shook at the cosmic lust reflected there. She could tell nothing about the creature's body. Its outline seemed to waver and alter subtly even as she looked at it; yet its substance was apparently solid enough. There was nothing misty or ghostly about it.
Compare this to the appearance of the idol of Tsathoggua from his temple in Commoriom:
He was very squat and pot-bellied, his head was more like a monstrous toad than a deity, and his whole body was covered with an imitation of short fur, giving somehow a vague sensation of both the bat and the sloth. His sleepy lids were half-lowered over his globular eyes; and the tip of a queer tongue issued from his fat mouth.
Or to what Ralibar Vooz saw underneath Mount Voormithadreth in "The Seven Geases" (although that was written after "The Slithering Shadow"):
A horror came upon him therewith; for, looking down, he beheld lying before the shadowy monster the lean husk of a thing that was neither man, beast nor Voormi. He stood hesitant, fearinig to go closer yet powerless to retreat. But, admonished by an angry hissing from the archaeopteryx, together with a slashing stroke of its beak between his shoulder-blades, he went forward till he could see the fine dark fur on the dormant body and sleepily porrected head.
...
There was a sluggish inclination of the toad-like head; and the eyes opened a little wider, and light flowed from them in viscous tricklings on the creased underlids.
When Conan fights the thing, its body resembles the formless spawn that served Tsathoggua:
He could not tell whether he was slashing off its members or whether he was cleaving its bulk, which knit behind the slicing blade. He was tossed to and fro in the violence of that awful battle, and had a dazed feeling that he was fighting not one, but an aggregation of lethal creatures. The thing seemed to be biting, clawing, crushing and clubbing him all at the same time. He felt fangs and talons rend his flesh; flabby cables that were yet hard as iron encircled his limbs and body, and worse than all, something like a whip of scorpions fell again and again across his shoulders, back and breast, tearing the skin and filling his veins with a poison that was like liquid fire.
Also, it is notable that Conan does not kill Thog. Usually, the Cimmerian would destroy whatever eldritch monsters he encountered, but Thog was merely wounded and driven off, as if it would not be appropriate for Howard's protagonist to slay another writer's creation.
Tsathoggua was popular among other mythos writers as well. H. P. Lovecraft included mention of Tsathoggua in "The Mound," although it does not have any particular importance to the plot, being merely an allusion to "The Tale of Satampra Zeiros" (including mention of the formless spawn again). Tsathoggua's appearances in other Lovecraft stories are in a similar vein.
Is there any specific indication, perhaps in authors' letters, that Thog was meant to be Tsathoggua?
