| Martial arts adept | 3 | Humanoid (any race) | Medium | 16 | 60 | | any alignment | | Volo's Guide to Monsters | | | | | | |
| Marut | 25 | Construct (inevitable) | Large | 22 | 432 | fly | lawful neutral | | Mordenkainen's Tome of Foes | | | | | | |
| Master of souls | 4 | Humanoid (human) | Medium | 12 | 45 | | neutral evil | | Adventures (Descent into Avernus) | | | | | | |
| Master thief | 5 | Humanoid (any race) | Medium | 16 | 84 | | any alignment | | Volo's Guide to Monsters | | | | | | |
| Mastiff | 0.12 | Beast | Medium | 12 | 5 | | unaligned | | Monster Manual (SRD) | 13 | 14 | 12 | 3 | 12 | 7 |
| Maurezhi | 7 | Fiend (demon) | Medium | 15 | 88 | | chaotic evil | | Mordenkainen's Tome of Foes | | | | | | |
| Maw demon | 1 | Fiend (demon) | Medium | 13 | 33 | | chaotic evil | | Volo's Guide to Monsters | | | | | | |
| Meazel | 1 | Humanoid (meazel) | Medium | 13 | 35 | | neutral evil | | Mordenkainen's Tome of Foes | | | | | | |
| Medusa | 6 | Monstrosity | Medium | 15 | 127 | | lawful evil | | Monster Manual (SRD) | 10 | 15 | 16 | 12 | 13 | 15 |
| Meenlock | 2 | Fey | Small | 15 | 31 | | neutral evil | | Volo's Guide to Monsters | | | | | | |
| Merfolk | 0.12 | Humanoid (merfolk) | Medium | 11 | 11 | swim | neutral | | Monster Manual (SRD) | 10 | 13 | 12 | 11 | 11 | 12 |
| Merregon | 4 | Fiend (devil) | Medium | 16 | 45 | | lawful evil | | Mordenkainen's Tome of Foes | | | | | | |
| Merrenoloth | 3 | Fiend (yugoloth) | Medium | 13 | 40 | swim | neutral evil | | Mordenkainen's Tome of Foes | | | | | | |
| Merrow | 2 | Monstrosity | Large | 13 | 45 | swim | chaotic evil | | Monster Manual (SRD) | 18 | 10 | 15 | 8 | 10 | 9 |
| Mezzoloth | 5 | Fiend (yugoloth) | Medium | 18 | 75 | | neutral evil | | Monster Manual (BR) | 18 | 11 | 16 | 7 | 10 | 11 |
| Mimic | 2 | Monstrosity (shapechanger) | Medium | 12 | 58 | | neutral | | Monster Manual (SRD) | 17 | 12 | 15 | 5 | 13 | 8 |
| Mind flayer | 7 | Aberration | Medium | 15 | 71 | | lawful evil | | Monster Manual | | | | | | |
| Mindwitness | 5 | Aberration | Large | 15 | 75 | fly | lawful evil | | Volo's Guide to Monsters | 10 | 14 | 14 | 15 | 15 | 10 |
| Minotaur | 3 | Monstrosity | Large | 14 | 76 | | chaotic evil | | Monster Manual (SRD) | 18 | 11 | 16 | 6 | 16 | 9 |
| Minotaur skeleton | 2 | Undead | Large | 12 | 67 | | lawful evil | | Monster Manual (SRD) | 18 | 11 | 15 | 6 | 8 | 5 |
| Moloch | 21 | Fiend (devil) | Large | 19 | 253 | | lawful evil | Legendary | Mordenkainen's Tome of Foes | 26 | 19 | 22 | 21 | 18 | 23 |
| Molydeus | 21 | Fiend (demon) | Huge | 19 | 216 | | chaotic evil | | Mordenkainen's Tome of Foes | | | | | | |
| Monodrone | 0.12 | Construct | Medium | 15 | 5 | fly | lawful neutral | | Monster Manual | | | | | | |
| Morkoth | 11 | Aberration | Medium | 17 | 130 | swim | chaotic evil | | Volo's Guide to Monsters | | | | | | |
| Mouth of grolantor | 6 | Giant (hill giant) | Huge | 14 | 105 | | chaotic evil | | Volo's Guide to Monsters | | | | | | |