| # | Name | Origin | Meaning | Gender | |
|---|---|---|---|---|---|
| 501 | Mihangel | Welsh | Welsh form of Michael 'God like'. | M | |
| 502 | Mil | Welsh | Legendary son of Dugum. | M | |
| 503 | Moesen | Welsh | Welsh form of Moses 'from the water'. | M | |
| 504 | Morcan | Welsh | Dwells near the sea. | M | |
| 505 | Morcar | Welsh | Dwells near the sea. | M | |
| 506 | Mordwywr | Welsh | Sailor. | M | |
| 507 | Moren | Welsh | Legendary son of Iaen. | M | |
| 508 | Morgan | Welsh | Bright sea. Dwells near the sea. | M | |
| 509 | Morgannwg | Welsh | From Glamorgan. | M | |
| 510 | Morgen | Welsh | Of the sea. Surname. | M | |
| 511 | Morthwyl | Welsh | Hammer. | M | |
| 512 | Morvran | Welsh | Legendary son of Tegid. | M | |
| 513 | Myrddin | Welsh | Welsh form of Merlin: Hawk. From the sea fortress. | M | |
| 514 | Myrick | Welsh | Dark-skinned; A Moor. Form of Maurice. | M | |
| 515 | Naw | Welsh | Legendary son of Seithved. | M | |
| 516 | Neb | Welsh | Legendary son of Caw. | M | |
| 517 | Neifion | Welsh | Welsh form of Neptune 'god of the seas'. | M | |
| 518 | Nerth | Welsh | Legendary son of Cadam. | M | |
| 519 | Nerthach | Welsh | Legendary son of Gwawrddur. | M | |
| 520 | Neued | Welsh | Legendary father of Tringad. | M | |