The green man, although he looks a bit monstrous, is actually a good guy who looks after the countryside.
He is depicted as a horned being peering out of foliage, usually oak leaves. He is also known as Jack-in the Green, Green George and Green Jack. You may see him accompanying a troop of morris dancers at folk music gatherings in Britain. He's the one who looks like a walking tree.
It is believed he has rainmaking powers to bring verdant meadows and well-fed livestock. He was frequently depicted in medieval art, including church decorations.
In various festivals, the green man is dunked in a river or pond to ensure that there will be enough rain to make the meadows and pastures green.