| Name | CR | Type | Environment |
| Magma Ooze | 7 | ooze (fire) | any underground |
| Magmin | 3 | outsider (elemental, extraplanar, fire) | any land (Plane of Fire) |
| Mahadatari | 19 | humanoid (kasatha) | any |
| Many-miened One | 19 | construct (magical) | any land |
| Maraquoi, Maraquoi Hunter | 1/2 | humanoid (maraquoi) | any (Marata) |
| Maraquoi, Maraquoi Shaman | 8 | humanoid (maraquoi) | any (Marata) |
| Marooned One | 8 | undead | any |
| Marrowblight | 7 | undead | any ruins (Eox) |
| Megalonyxa | 4 | monstrous humanoid | any |
| Mementor | 13 | fey (extraplanar) | any (Shadow Plane) |
| Memetic Zenith | 15 | outsider (good, technological) | any urban |
| Memory Muck | 9 | ooze | any |
| Mephit | 3 | outsider (extraplanar) | any (Elemental Planes) |
| Meteorite Drake | 16 | magical beast (earth) | any high elevation |
| Midships Mimic | 8 | aberration (shapechanger) | any (starship) |
| Mi-Go | 6 | plant | any |
| Mi-Go, Mi-Go High Priest | 11 | plant | any |
| Mindscape Mindreaper | 8 | monstrous humanoid | any mindscape |
| Mohrg | 8 | undead | any |
| Mole-Bettle | | vermin | any underground (Weydana-4) |
| Moonflower | 8 | plant | any land |
| Moonflower, Moonflower Titan | 16 | plant | any land |
| Mordagast, Butcher | 13 | animal | any forest, hills, or vacuum |
| Mordagast | 7 | animal | any forest or hills |
| Morlamaw | 3 | monstrous humanoid (aquatic) | any water (Arniselle) |
| Morlamaw, Morlamaw Mayor | 9 | monstrous humanoid (aquatic) | any water (Arniselle) |
| Mountain Eel | 6 | animal | temperate or warm mountains (Castrovel) |
| Moyishuu | 7 | fey | any (Veyvilla-6) |
| Mucilaginous Cloud | 5 | ooze | any sky |
| Muneen | 11 | magical beast | any (Chonax) |
| Muotta | 2 | animal | any sky (Utraneus) |
| Murzzilat, Bantling | 1 | animal (aquatic) | any underground or water |
| Murzzilat | 4 | animal (aquatic) | any underground or water |