Cave beasts are a species of humanoid creatures in the Cthulhu mythos; believed to be the inbred descendents of cannibalistic cave-dwelling humans.

They are described as white semi-humanoid creatures with long clawed nails.

Most of the time, the tread seemed to be that of a quadruped, walking with a singular lack of unison betwixt hind and fore feet, yet at brief and infrequent intervals I fancied that but two feet were engaged in the process of locomotion. [...] Its hair was snow-white, a thing due no doubt to the bleaching action of a long existence within the inky confines of the cave, but it was also surprisingly thin, being indeed largely absent save on the head, where it was of such length and abundance that it fell over the shoulders in considerable profusion.
From the tips of the fingers or toes long nail-like claws extended. The hands or feet were not prehensile, a fact that I ascribed to that long residence in the cave which, as I before mentioned, seemed evident from the all-pervading and almost unearthly 
whiteness so characteristic of the whole anatomy. No tail seemed to be present.
They were black, those eyes, deep, jetty black, in hideous contrast to the snow-white hair and flesh. Like those of other cave denizens, they were deeply sunken in their orbits, and were entirely destitute of iris. As I looked more closely, I saw that they were set in a face less prognathous than that of the average ape, and infinitely more hairy. The nose was quite distinct.
The creature I had killed, the strange beast of the unfathomed cave was, or had at one time been, a MAN!!!
~ H.P. Lovecraft , "The Beast in the Cave"

When Dr. Trask, the anthropologist, stooped to classify the skulls, he found a degraded mixture which utterly baffled him. They were mostly lower than the Piltdown man in the scale of evolution, but in every case definitely human. Many were of higher grade, and a very few were the skulls of supremely and sensitively developed types. All the bones were gnawed, mostly by rats, but somewhat by others of the half-human drove. [...] It was the antechamber of hell, and poor Thornton fainted again when Trask told him that some of the skeleton things must have descended as quadrupeds through the last twenty or more generations.

Horror piled on horror as we began to interpret the architectural remains. The quadruped things—with their occasional recruits from the biped class—had been kept in stone pens, out of which they must have broken in their last delirium of hunger or rat-fear. There had been great herds of them, evidently fattened on the coarse vegetables whose remains could be found as a sort of poisonous ensilage at the bottom of huge stone bins older than Rome. I knew now why my ancestors had had such excessive gardens—would to heaven I could forget! The purpose of the herds I did not have to ask.

~ H.P. Lovecraft , "The Rats in the Walls"

The thing will haunt me, for who can say the extermination is complete, and that analogous phenomena do not exist all over the world? Who can, with my knowledge, think of the earth's unknown caverns without a nightmare dread of future possibilities? I cannot see a well or a subway entrance without shuddering . . . why cannot the doctors give me something to make me sleep, or truly calm my brain when it thunders?

What I saw in the glow of flashlight after I shot the unspeakable straggling object was so simple that almost a minute elapsed before I understood and went delirious. The object was nauseous; a filthy whitish gorilla thing with sharp yellow fangs and matted fur. It was the ultimate product of mammalian degeneration; the frightful outcome of isolated spawning, multiplication, and cannibal nutrition above and below the ground; the embodiment of all the snarling and chaos and grinning fear that lurk behind life. It had looked at me as it died, and its eyes had the same odd quality that marked those other eyes which had stared at me underground and excited cloudy recollections. One eye was blue, the other brown. They were the dissimilar Martense eyes of the old legends, and I knew in one inundating cataclysm of voiceless horror what had become of that vanished family; the terrible and thunder-crazed house of Martense.

~ H.P. Lovecraft , "The Lurking Fear"

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