This list of mythical creatures that begin with this letter and I am constantly gathering information from each continent, and is continually being updated.
Merrow (Irish mythology and Scottish)
The merrows supposedly require a magical cap in order to travel between deep water and dry land. The term appears in two tales set in Ireland published in the 19th century: "Lady of Gollerus", where a green-haired merrow weds a local Kerry man who deprives her of the "magical red cap" (cohuleen druith); and "The Soul Cages" where a green-bodied grotesque male merrow entertains a fisherman at his home under the sea.
A number of other terms in Irish are used to denote a mermaid or sea-nymph, some tracing back to mythological tracts from the medieval to the post-medieval period. The Middle Irish murdúchann is a siren-like creature encountered by legendary ancestors of the Irish.