This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Mermaid/Merman (multiple cultures)
In folklore, a mermaid is an aquatic creature with the head and upper body of a female human and the tail of a fish. Mermaids appear in the folklore of many cultures worldwide, including Europe, Asia, and Africa. Mermaids are sometimes associated with perilous events such as floods, storms, shipwrecks, and drownings. In other folk traditions, they can be benevolent or beneficent, bestowing boons or falling in love with humans.
The male equivalent of the mermaid is the merman, also a familiar figure in folklore and heraldry. Although traditions about and sightings of mermen are less common than those of mermaids, they are generally assumed to co-exist with their female counterparts. The male and the female collectively are sometimes referred to as merfolk or merpeople.