Three entries with the name Maine.
Maine
Maines, Mainet, Meines, Mene, Moine, Moyne
In the Vulgate Merlin, Arthur’s great uncle, apparently the oldest, of King Constans of England, Arthur’s grandfather, who preceded Vortigern as King of Britain. He is called Constans in the chronicles.
The son of Constantine and Ivoire, and brother of Uther and Pendragon, Maine was foisted to the throne by Vortigern after his father’s death. He was an impotent king, and Vortigern eventually had him assassinated, killed by a dozen barons who wanted Vortigern for their king. Maine survived and was avenged by his brothers.
The alternate name for Constans undoubtedly comes from his title in the chronicles: le Moine, or “the Monk.”
Sources
Vulgate Merlin | 1220-1235
Unknown (the pre-Arthurian period) | Baudin Butor, c. 1290
Arthour and Merlin | Late 13th century
Maine
A region of northwest France, south of Normandy.
According to Geoffrey of Monmouth, it was part of Arthur’s domain, and was ruled under Arthur by Borel.
Source
Historia Regum Britanniae | Geoffrey of Monmouth, c. 1138
Maine
A river in Scotland, bordering the lands of the King of the Land Beyond the Borders of Galone.
Arthur and the King fought a battle near the river.
See also
Matrona | The Legend of King Arthur
Source
Lancelot do Lac | 1215-1220