Anthony north: When we think of monsters we automatically think of Dragons, of maybe King Kong, or perhaps the Loch Ness Monster. Through myth, media or mystery, such monsters almost become part of our psyche. However, there is another category of monster -creatures, or even strange people, who seem to invade the world briefly, and then disappear, becoming nothing but a puzzle at the extremities of paranormal literature. Let's have a few examples. Demon and Gasser: One night in April 1977 a teenager was driving near Dover, Massachusetts, when he saw an entity with large head, protruding eyes, long, thin limbs and peach-coloured skin. Two hours later, another teenager saw the same entity. The following night, what became known as the Dover Demon was seen by another teenager for one last time. Researchers subsequently matched the entity to the pygmy Mannegishi, a mythological creature believed in by the nearby Cree Native Americans. Several decades earlier, in September 1944, the residents of Mattoon, Illinois, were terrorised for nearly a fortnight by a Mad Gasser, a tall, dark-clad man with a tight-fitting hat. First seen as a shadowy figure outside houses, the gasser eventually squirted something into people’s bedrooms, resulting in temporary paralysis. Mothman: The Mothman terrorised Point Pleasant in West Virginia for several years in the 1960s. A grey, tall creature with wings, no head, human legs and red eyes in its chest, it was seen on over a hundred occasions. Some researchers associated it with ‘Big-hoot’, a Native American legendary monster, whilst journalist John Keel, who investigated it in the 1970s, associated it with the UFO phenomenon.So engrossed in the case was Keel that he began hearing voices, and all kinds of phenomena exploded around him. Spring Heeled Jack: Perhaps the most famous such entity was Spring Heeled Jack, who terrorized Britain from 1837 to 1904. Described as a cloaked figure with red eyes, pointed ears and talons, he could breathe fire and jump over houses.