| Name | CR | Type | Environment |
| 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) |
| Mi-Go | 6 | plant | any |
| Mi-Go, Mi-Go High Priest | 11 | plant | any |
| Mohrg | 8 | undead | any |
| Moonflower | 8 | plant | any land |
| Moonflower, Moonflower Titan | 16 | plant | any land |
| Mountain Eel | 6 | animal | temperate or warm mountains (Castrovel) |
| Mucilaginous Cloud | 5 | ooze | any sky |