Here is how the Zuni creation myth describes the ancestors of the Zuni people:
Men and creatures were more alike then than now. Our fathers were black, like the caves they came from; their skins were cold and scaly like those of mud creatures; their eyes were goggled like an owl’s; their ears were like those of cave bats; their feet were webbed like those of walkers in wet and soft places; they had tails, long or short, as they were old or young. Men crouched when they walked, or crawled along the ground like lizards. They feared to walk straight, but crouched as before time they had in their cave worlds, that they might not stumble or fall in the uncertain light.
Source: Katharine Berry Judson (ed.), Myths and Legends of California and the Old Southwest (Chicago: A. C. McClurg & Co., 1912), 24.
Now, let's look at how Lovecraft describes that Deep Ones in Shadow Over Innsmouth:
I think their predominant colour was a greyish-green, though they had white bellies. They were mostly shiny and slippery, but the ridges of their backs were scaly. Their forms vaguely suggested the anthropoid, while their heads were the heads of fish, with prodigious bulging eyes that never closed. At the sides of their necks were palpitating gills, and their long paws were webbed. They hopped irregularly, sometimes on two legs and sometimes on four. […] Their croaking, baying voices, clearly used for articulate speech, held all the dark shades of expression which their staring faces lacked.
Aside from the matter of color (a regional variation, perhaps?), these creatures are identical.
Of course we all know there is no such thing as Deep Ones. This forces us to ask the question: If we can find fictional creatures in "ancient texts" and myths using the methods of alternative history, what possible basis is there for assuming the aliens and Atlanteans and Muvians alternative historians find in them really exist?