Fiend Without A Face (1958) is a genuinely creepy and eerie and scary movie. it's even succeeds when the monster finally become visible, due to some excellent stop-motion effects and nice full size props.
of course, the fiends are invisible for the first 2/3 of the movie, attacking people and sucking their brains out with some of the grossest sound effects you could possibly imagine. I saw the first part of this movie when I was quite young, and since there was no visible monster I wasn't that interested.
the animation by Ruppel & Nordhoff
really makes the fiends unusual, and helps make the movie stand out. it's the surrealism that does it, though at the time they thought this was the most "realistic" effect they could offer. this film has been released by Criterion and is an excellent disc.