Another film I found a bit frightening was the 1974 Larry Cohen movie, It's Alive. The plot revolves around a couple who are expecting a new baby. When the baby is born, it turns out to be a hideously fanged mutant with claws that kills when frightened.
As ridiculous as the story sounds, the film manages to be very creepy and suspenseful. I think that's mostly due to us never really getting see what the infant looks like. We are only given glimpses of its bulbous head and fangs. On top of that, the sounds that it makes when someone encounters it are just plain disturbing.