Just to add to the pot - I've experienced it only twice, but both times I experienced no hallucinations at all. The first time, the room was dark and I knew there was "something out there" waiting to get me and I had to switch on the light to see it, but couldn't move. The second time there was nothing, but I was terrified anyway. Both times I managed to wake myself up (eventually).

I can quite imagine, however, that our dreaming mind might try to put a face on the stalking horror. Given you're already asleep and just out of REM state, there's no surprise in extra visual hallucinations here - and of course they'd be relevant to your own cultural experiences.