This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Jack-o'-lantern (Medieval folklore)
A jack-o'-lantern (or jack o'lantern) is a carved lantern, most commonly made from a pumpkin, or formerly a root vegetable such as a mangelwurzel, rutabaga or turnip. Jack-o'-lanterns are associated with the Halloween holiday. Its name comes from the phenomenon of strange lights flickering over peat bogs, called jack-o'-lanterns (also known as, will-o'-the-wisps).
An old Irish folk tale from the mid-18th century tells of Stingy Jack, a lazy yet shrewd blacksmith who uses a cross to trap Satan. One story says that Jack tricked Satan into climbing an apple tree, and once he was up there, Jack quickly placed crosses around the trunk or carved a cross into the bark, so that Satan could not get down.