Sleep paralysis can be a really interesting subject. For one, what you dream about often depends on what you're scared of(in my case, it was being stuck in a fire and eventually having to breathe in the fire). A few cultural ones have already been mentioned in this thread: alien abduction, terrorist attacks, plane crashes, old hag, demons of varying nature. I remember a Japanese friend telling me that in Japan, it is often described as an old woman with a lantern and a knife standing by your bed while you cannot move and that it is called 'kanashibari', which I can only translate as 'tied up in metal'. In the Philippines there is a phenomenon known as 'bangungut', which is a demon sitting on your chest and pushing the air out of your lungs. And I'm sure at least some of you have heard of a succubus(or the male variant, incubus), which in some historic descriptions were also known to sit on people's chests as they slept. Even the word "nightmare" finds its roots in such a demonic creature.
If you then consider that, for example, alien abduction reports didn't start until humanoid grey alien media were created and it depends on where you come from what kind of form the "ominous person strangling or crushing you" takes, you can clearly see that it's all between your ears and that you are basically creating your own experience by drawing from your subconscious fears and/or traumatic experiences.
It's also quite intriguing to note that a lot of other symptoms and triggers are similar in people who experience sleep paralysis. The feeling of the body being out of proportion, abdominal pressure unrelated to chest pressure, incoherent colour observance, stressful times before an episode, watching a scary film, having to endure long periods of high adrenaline levels, a consistent lack of sleep resulting in a physical shutdown, etc.
Sometimes it is kind of funny to see what our brains get up to when you can isolate one area of them and have the rest at idle.