pareidoliacs and people experiencing "sleep paralysis" might see them more frequently. i have, a few times, when i was younger. i also have a friend who described something quite similar, only more angry and violent, though he came from a violent home originally, so that may have affected his experience some.
interestingly, some say that they may be where the term "nightmare" came to be coined, since back in the day, people were plagued by shadowy figures such as horses in the night when most were sleeping, and other people attributed it to "bad dreams"...
all-in-all, they cannot have more power over you than you LET them, though it may seem different at times. they exist in the minds of those who are troubled, most frequently, and especially to those who are on the fringe of sanity, to push them over the edge.
...you might find this interesting:
http://mrvoodoo.hubpages.com/hub/Sleep_P…