The Cave Girl by Edgar Rice Burroughs

The dim shadow of the thing was but a blur against the dim shadows of the wood behind it.
The young man could distinguish no outline that might mark the presence as either brute or human.
He could see no eyes, yet he knew that somewhere from out of that noiseless mass stealthy eyes were fixed upon him.
This was the fourth time that the thing had crept out from the wood as darkness was settling—the fourth time during those three horrible weeks since he had been cast upon that lonely shore that he had watched, terror-stricken, while night engulfed the shadowy form that lurked at the forest’s edge.
It had never attacked him, but to his distorted imagination it seemed to slink closer and closer as night fell—waiting, always waiting for the moment that it might find him unprepared.