The most common defining feature of sleep paralysis is an imagined cause of the paralysis such as something sitting on your chest. The mare is probably the most iconic myth of this type, but being sat upon, ridden, entangled in hair and so on all make a showing. Why we see humanoid monsters in these delusions probably has something to do with other mental phenomena like shadow people or prisoner's cinema. With shadow people we seek to rationalize emotional responses by generating a human cause so the random anxiety that one is being followed manifests as followers. With prisoner's cinema a person deprived of light for long enough seems to have some bleeding stimulus from other parts of the brain into visual processing, people translate this bleed into sounds, colors, patterns, and the human form.