This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Marmennill (Norse mythology)
In Norse mythology, the Marmennill were mermen with the ability to prophesize the future. Its female counterpart was the Margygur. It was a marine male creature usually represented as having the head, trunk, and arms of a man and a lower part like the tail of a fish.
Marmennil (merman) is a creature that is said to live at the bottom of the sea. He looks like a human but is smaller and has long fingers. He likes to tease fishermen by biting of the bait off their hooks and attaching the hooks to the bottom of the sea so the line will break.