Just to have a running count, here's a list based on all the monsters featured in Desert of Desolation. (I'm not including non-monsters like "Blood of Vol Fanatic.")
Angel (Angel of Vengeance)
Animated Object (Animated Statue)
Astral Stalker (Astral Stalker)
Demon (Bar-Lgura)
Blade Spider (Blade Spider)
Cyclops (Cyclops)
Devil (Osyluth, Spined Devil)
Dinosaur (Macetail Behemoth)
Dragon (Capricious Copper Dragon)
Drake (Rage Drake)
Drider (Drider)
Drow (Drow Blademaster, Drow Spider Priestess )
Dryad (Black Woods Dryad)
Elemental (Fire Archon, Fire Elemental)
Ettercap (Ettercap Webspinner)
Flame Snake (Flame Snake)
Ghoul (Ravenous Ghoul)
Harpy (Shrieking Harpy)
Manticore (Manticore Sniper)
Mummy (Guardian Mummy)
Naga (Naga)
Nightmare (Nightmare)
Ogre (Ogre Brute)
Sahuagin (Sahuagin, Sahuagin Baron)
Shadar-Kai (Shakar-Kai Assassin)
Shadow Mastiff (Shadow Mastiff)
Skeleton (Boneshard Skeleton)
Sphinx (Sphinx)
Swarm (Demonweb Swarm, Rot Scarab Swarm)
Thundertusk Boar (Thundertusk Boar)
Troll (Feral Troll)
Umber Hulk (Umber Hulk Delver)
Visejaw Crocodile (Visejaw Crocodile )
Werewolf (Werewolf Champion)
Yuan-Ti (Yuan-Ti Malison, Yuan-Ti Champion of Zehir)
I don't know if "souped-up animals" like the Flame Snake, Thundertusk Boar and Visejaw Crocodile will each get their own page or if they'll be grouped together like dire animals.
Aside from this list, we know that Orcus, "foulspawn," beholders and gnomes are in the MM.
That covers a substantial portion of the 300 monsters (if you assume that includes every stat block for monster categories like dragons). If you add obvious choices like goblin, orc and kobold, you ought to be able to come up with something very much like the final list!
Angel (Angel of Vengeance)
Animated Object (Animated Statue)
Astral Stalker (Astral Stalker)
Demon (Bar-Lgura)
Blade Spider (Blade Spider)
Cyclops (Cyclops)
Devil (Osyluth, Spined Devil)
Dinosaur (Macetail Behemoth)
Dragon (Capricious Copper Dragon)
Drake (Rage Drake)
Drider (Drider)
Drow (Drow Blademaster, Drow Spider Priestess )
Dryad (Black Woods Dryad)
Elemental (Fire Archon, Fire Elemental)
Ettercap (Ettercap Webspinner)
Flame Snake (Flame Snake)
Ghoul (Ravenous Ghoul)
Harpy (Shrieking Harpy)
Manticore (Manticore Sniper)
Mummy (Guardian Mummy)
Naga (Naga)
Nightmare (Nightmare)
Ogre (Ogre Brute)
Sahuagin (Sahuagin, Sahuagin Baron)
Shadar-Kai (Shakar-Kai Assassin)
Shadow Mastiff (Shadow Mastiff)
Skeleton (Boneshard Skeleton)
Sphinx (Sphinx)
Swarm (Demonweb Swarm, Rot Scarab Swarm)
Thundertusk Boar (Thundertusk Boar)
Troll (Feral Troll)
Umber Hulk (Umber Hulk Delver)
Visejaw Crocodile (Visejaw Crocodile )
Werewolf (Werewolf Champion)
Yuan-Ti (Yuan-Ti Malison, Yuan-Ti Champion of Zehir)
I don't know if "souped-up animals" like the Flame Snake, Thundertusk Boar and Visejaw Crocodile will each get their own page or if they'll be grouped together like dire animals.
Aside from this list, we know that Orcus, "foulspawn," beholders and gnomes are in the MM.
That covers a substantial portion of the 300 monsters (if you assume that includes every stat block for monster categories like dragons). If you add obvious choices like goblin, orc and kobold, you ought to be able to come up with something very much like the final list!