H.P. Lovecraft 
The Beast in the Cave 

A man touring a Mammoth Cave separates from his guide and becomes lost. His torch expires and he is giving up hope of finding a way out in the pitch dark, when he hears strange non-human footsteps approaching him. Thinking it to be a lost mountain lion or other such beast, he picks up a stone and throws it toward the source of the sound. The beast is hit and crumples to the floor. The guide finds the protagonist, and together they examine the fallen creature with the guide’s torchlight. The creature mutters in its last breaths and they see its face, discovering that it is in fact a pale, deformed human, who had also become lost in the cave many years ago.