| Name | CR | Type | Environment |
| Maenad | 8 | monstrous humanoid | temperate forests or plains |
| Maftet | 6 | monstrous humanoid | temperate and warm deserts and mountains |
| Magma Ooze | 7 | ooze (fire) | any volcano or underground |
| Magmin | 3 | outsider (elemental, extraplanar, fire) | any land (Plane of Fire) |
| Makara Vahana | 9 | magical beast (aquatic, augmented animal) | any |
| Malbolgian Cerberi | 12 | outsider (evil, extraplanar, lawful) | any (Hell) |
| Mamiwa | 2 | outsider (air, elemental, native, water) | any |
| Mana Wastes Mutant (Gnoll) | 4 | aberration (augmented humanoid, gnoll) | warm plains or deserts (the Mana Wastes) |
| Manananggal | 7 | undead | any land |
| Manasaputra, Maharishi Manu | 16 | outsider (extraplanar, good, lawful, manasaputra) | any (Material Plane or Positive Energy Plane) |
| Manasaputra, Manu | 12 | outsider (extraplanar, good, lawful, manasaputra) | any (Material Plane or Positive Energy Plane) |
| Manasaputra, Rishi Manu | 14 | outsider (extraplanar, good, lawful, manasaputra) | any (Material Plane or Positive Energy Plane) |
| Manasaputra, Twilight Pitri (Barhisad) | 18 | outsider (extraplanar, good, lawful, manasaputra) | any (Positive Energy Plane) |
| Mandragora | 4 | plant | cold or temperate forests |
| Man-Eating Animal, Man-Eating Aurochs | 3 | magical beast | temperate plains |
| Manitou | 15 | outsider (native) | any plains or forests |
| Manticore | 5 | magical beast | warm hills and marshes |
| Mantis, Deadly Mantis | 11 | vermin | tropical jungles (Mediogalti Island) |
| Mantis, Giant Mantis | 3 | vermin | temperate forests |
| Mapinguari | 11 | magical beast | warm forests |
| Marax | 11 | animal | warm forests or plains |
| Mederach | 9 | outsider (chaotic, extraplanar, good) | any (Elysium) |
| Medusa | 7 | monstrous humanoid | temperate marshes and underground |
| Medusa, Eygreas | 13 | monstrous humanoid | — |
| Megafauna, Archelon | 5 | animal | warm or temperate water or coastlines |
| Megafauna, Arsinoitherium | 7 | animal | temperate plains |
| Megafauna, Baluchitherium | 8 | animal | warm forests |
| Megafauna, Basilosaurus | 12 | animal | any oceans |
| Megafauna, Brontotherium | 8 | animal | any plains |
| Megafauna, Chalicotherium | 6 | animal | temperate forests |
| Megafauna, Deinotherium | 7 | animal | any forests or plains |
| Megafauna, Elasmotherium | 9 | animal | temperate forests or plains |
| Megafauna, Embolotherium | 5 | animal | any plains |
| Megafauna, Glyptodon | 6 | animal | temperate plains |
| Megafauna, Kaprosuchus | 5 | animal | warm forests |
| Megafauna, Megalania | 7 | animal | warm forests or plains |
| Megafauna, Megaloceros | 4 | animal | cold or temperate forest and plains |
| Megafauna, Megatherium | 5 | animal | temperate or warm forest |
| Megafauna, Moa | 2 | animal | warm plains |
| Megafauna, Titanoboa | 13 | animal | warm swamps |
| Megafauna, Uintatherium | 7 | animal | temperate swamps |
| Melacage | 5 | undead (incorporeal) | any |
| Melfesh Monster | 6 | plant (extraplanar) | any (Varisia) |
| Menotherian | 15 | outsider (chaotic, elf, extraplanar, shapechanger) | any |
| Mephit | 3 | outsider (varies) | any (elemental planes) |
| Mercane | 5 | outsider (extraplanar) | any land or underground |
| Merfolk | 1/3 | humanoid (aquatic) | temperate ocean |
| Merlucent | 3 | aberration (aquatic) | any ocean |
| Merrow, Freshwater Merrow | 3 | humanoid (aquatic, giant) | temperate lakes or rivers |
| Merrow, Saltwater Merrow | 6 | humanoid (aquatic, giant) | temperate oceans |
| Mezlan | 14 | ooze (shapechanger) | any |
| Miengu | 11 | fey (aquatic) | any aquatic |
| Mi-go | 6 | plant | any |
| Mihstu | 8 | outsider (air, elemental, extraplanar) | any (Plane of Air) |
| Mimic | 4 | aberration (shapechanger) | any |
| Mimic Ooze | 9 | ooze | any land |
| Mindslaver Mold | 3 | plant | any |
| Mindspin Ram | 2 | animal | cold mountains |
| Minotaur | 4 | monstrous humanoid | temperate ruins or underground |
| Minotaur, Labyrinth Guardian | 8 | undead (augmented monstrous humanoid) | temperate ruins or underground |
| Minotaur, Labyrinth Minotaur | 16 | outsider (chaotic, evil, extraplanar) | any (Ivory Labyrinth) |
| Minotaur, Minotaur Marauder | 6 | monstrous humanoid | temperate ruins or underground |
| Minotaur, Prophet of Baphomet | 13 | monstrous humanoid | temperate ruins or underground |
| Minotaur, Tangle Tender | 10 | monstrous humanoid | temperate ruins or underground |
| Mire Nettle | 2 | plant | temperate swamps |
| Mirror Man | 5 | construct | any land |
| Mirror Serpent | 5 | magical beast (aquatic) | any coastlines |
| Mite | 1/4 | fey | any underground |
| Mngwa | 7 | magical beast | warm forests |
| Mobogo | 10 | magical beast (aquatic) | temperate swamps |
| Mockingfey | 1 | fey
| any forests |
| Mohrg | 8 | undead | any |
| Moit of Shub-Niggurath, Spawning Canker | 9 | aberration | any land |
| Mokele-Mbembe | 9 | animal | tropical lakes or rivers |
| Moldwretch | 7 | plant | any swamps or underground |
| Mome Rath | 2 | animal | temperate forests (First World) |
| Mongrel Giant, Mongrel Cave Giant | 6 | humanoid (giant) | — |
| Mongrel Giant, Mongrel Cave Giant | 7 | humanoid (giant) | any underground |
| Mongrelman | 1 | monstrous humanoid | any ruins or underground |
| Monkey Swarm | 2 | animal (swarm) | warm forests |
| Moon Dog | 9 | outsider (extraplanar, good) | any (Nirvana) |
| Moon-Beast | 11 | aberration | any land |
| Moonflower | 8 | plant | any land |
| Moose | 3 | animal | cold forests |
| Morgodea | 4 | fey | any urban |
| Morlock | 2 | monstrous humanoid | any underground |
| Mortic, Angheuvore | 2 | humanoid (elf, mortic) | any |
| Mortic, Etioling | 10 | humanoid (gnome, mortic) | any |
| Mortic, Gurgist | 6 | humanoid (human, mortic) | any |
| Mortic, Jitterbone | 4 | humanoid (halfling, mortic) | any |
| Mortic, Lifeleecher | 8 | humanoid (mortic, orc) | temperate hills, mountains, or underground |
| Mortic, Relictner | 12 | humanoid (dwarf, mortic) | any ruins |
| Mosquito, Giant Mosquito | 6 | vermin | temperate or tropical swamps |
| Mosquito, Mosquito Swarm | 3 | vermin (swarm) | tropical swamps |
| Mosslord | 18 | plant | temperate forests |
| Mothman | 6 | monstrous humanoid | any land |
| Mothman, Mothman Elder | 12 | monstrous humanoid | — |
| Mu Spore | 21 | plant | any |
| Muckdweller | 1/4 | magical beast | temperate marshes |
| Mudlord | 6 | outsider (earth, elemental, water) | temperate or warm marshes or underground |
| Muhuru | 10 | magical beast | warm forests |
| Mummified Animals, Mummified Baboon | 1/2 | undead | warm ruins |
| Mummified Animals, Mummified Crocodile | 2 | undead | warm ruins |
| Mummified Animals, Mummified Elephant | 9 | undead | warm ruins |
| Mummified Animals, Mummified Hippopotamus | 6 | undead | warm ruins |
| Mummified Creature, Mummified Gynosphinx | 9 | undead (augmented magical beast) | warm ruins |
| Mummy | 5 | undead | any |
| Mummy Lord | 10 | undead (augmented human) | any |
| Munavri | 1 | humanoid (munavri) | any underground |
| Murajau | 5 | monstrous humanoid (aquatic, extraplanar) | any water (Plane of Water) |
| Muse | 16 | fey | any |
| Mutant, Mutant Orc | 7 | aberration (orc) | any (Numeria) |
| Mutant, Mutant Orc | 7 | aberration (augmented orc) | any |
| Myceloid | 4 | plant | any underground |