The Green Man is a puzzling figure. No images have been found of any Green Man of any kind before 400 AD. It may, however have been a long-standing symbol that we just don't have any earlier examples of, or it may have been introduced by another culture or society. The fact that the figure is commonly carved on churches is puzzling, since it's generally assumed that the symbol is a fertility god or spirit. The carved or drawn figures exist in various forms all across Europe, in England, Germany, France, and even as far as Greece. It's one of the more mysterious medieval symbols, but there is a lot of really interesting research on it floating around out there. This site might have more information:
http://www.greenmanenigma.com/history.html