No. A Hag is an old woman/witch-esque-ish-person.

An Old Hag is simply "hallucinating" while you're in SP. You can't move, and you see a dark silhou..ette...something go through your room, scaring the freaking shit out of you if you don't know what's going on. If you try to get up, but you're in SP, something happens with your brains and blood and all that, and basically because of the SP you feel a huge weight on your chest. They can also hardly breathe, if they're panicing. That's when the people think the hag is actually sitting on them, suffocating them.

It's all in your head though.