A Night Hag is a type of malevolent creature described in many world cultures that will visit mortals during the hours of night and torment them via sitting on their chest and "riding" them, giving terrible nightmares, paralysis and even in some cases, death.
A Night Hag is one of many such nocturnal predators but is the most striking in terms of being described constantly across many media, though considered a type of demonic hag (or witch) in most folklore and fantasy settings the "Old Hag" has recently been explained as a hallucination caused by sleep paralysis.
Due to the frightening nature of sleep paralysis it has still become a common (if often tongue-in-cheek) trend to describe "sleep paralysis demons" - though most now consider them purely symbolic.
Other entities heavily associated with this phenomena are shadow people, stick men, nightmares, succubi, incubi and even alien abductions (especially those involving the Greys).