The evolutionary trait of the Uncanny Valley suggest that there used to be something that could mimic humans (worldbuilding-wise, not in real life.) I can see the Narrator (Black, non-human) scaring Stanley as a prank but then his instincts kick in and his brain thinks it saw a predator.
[EXTENSIVE LORE/WORLDBUILDING EXPLANATION BELOW]
More worldbuilding and lore will be posted in the future.