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-In-Irons (English)
Jack-In-Irons is a mythical giant of Yorkshire lore who haunts lonely roads. He is covered with chains and wears the heads of his victims. He wields a large, spiked club. His name may not be Jack as other Yorkshire folklore refers to "Jack in the Green" and more so the name Jack may just be a term for calling the person an unknown male, like John Doe today.
Typically large giant-like figures, they are traditionally clad in large clanking chains, and were reported to have the severed heads of enemies tied to his belt. His weapon of choice was usually a large spiked cudgel. Historically, they are supposed to have haunted lonely roads in Yorkshire.