Another theory suggest that the name Harlequin is taken from that of a mischievous “devil” or “demon” character in popular French passion plays. It originates with an Old French term herlequin, hellequin, first attested in the 11th century, by the chronicler Orderic Vitalis, who recounts a story of a monk who was pursued by a troop of demons when wandering on the coast of Normandy (France ) at night. These demons were led by a masked, club-wielding giant and they were known as familia herlequin (var. familia herlethingi).
Source: Harlequin – Wikipedia