Tome of Beasts is the first massive monster book. Not only is it an amazing resource of monsters, but the Open Game Content declaration is unusually broad as well. Whereas most monster books only put monster statistics under the OGL, Tome of Beasts puts the descriptions under it as well. Aside from removing some Product Identity in the descriptions, and removing all unique monsters (which have proper names), a majority of the content is provided here. The Demon Lords, Arch-devils, and Fey Lords and Ladies are the biggest chunks of missing monsters due to Product Identity, though there's other bits as well.
Since it's not clear if "NPCs" are "Monsters" or not, all of the Villain Codex in Appendix A is also excluded.
OGL Content:
Aboleth, Nihileth - (monster)
Abominable Beauty - (monster)
Accursed Defiler - (monster)
Ala - (monster)
Algorith - (monster)
Alseid - (monster)
Amphiptere - (monster)
Andrenjinyi - (monster)
Angatra - (monster)
Angel, Chained - (monster)
Angel, Fidele - (monster)
Angler Worm - (monster)
Ant, Giant - (monster)
Ant, Giant Queen - (monster)
Anubian - (monster)
Arborial Grappler - (monster)
Aridini - (monster)
Asasonbosam - (monster)
Azza Gremlin - (monster)
Baba Yagas Horsemen - (monster)
Bagiennik - (monster)
Bastet Temple Cat - (monster)
Bearfolk - (monster)
Behtu - (monster)
Beli - (monster)
Bereginyas - (monster)
Blemmyes - (monster)
Boloti - (monster)
Bone Collective - (monster)
Bone Crab - (monster)
Bone Swarm - (monster)
Bouda - (monster)
Broodiken - (monster)
Bucca - (monster)
Bukavac - (monster)
Buraq - (monster)
Burrowling - (monster)
Cactid - (monster)
Cambium - (monster)
Carrion Beetle - (monster)
Cavelight Moss - (monster)
Chelicerae - (monster)
Chernomoi - (monster)
Child Of The Briar - (monster)
Chronalmental - (monster)
Cikavak - (monster)
Clockwork Abomination - (monster)
Clockwork Beetle - (monster)
Clockwork Beetle Swarm - (monster)
Clockwork Hound - (monster)
Clockwork Huntsman - (monster)
Clockwork Myrmidon - (monster)
Clockwork Watchman - (monster)
Clockwork Weaving Spider - (monster)
Clurichaun - (monster)
Cobbleswarm - (monster)
Corposanti - (monster)
Corpse Mound - (monster)
Dau - (monster)
Death Butterfly Swarm - (monster)
Deathwisp - (monster)
Deep One - (monster)
Deep One Archimandrite - (monster)
Deep One Hybrid Priest - (monster)
Demon, Apau Perape - (monster)
Demon, Berstuc - (monster)
Demon, Kishi - (monster)
Demon, Malakbel - (monster)
Demon, Psoglav - (monster)
Demon, Rubezahl - (monster)
Derro Fetal Savant - (monster)
Derro Shadow Antipaladin - (monster)
Devil, Automata - (monster)
Devil, Chort - (monster)
Devil, Crystalline - (monster)
Devil, Guilded - (monster)
Devil, Ink - (monster)
Devil, Koralk - (monster)
Devil, Lunar - (monster)
Devil, Orobas - (monster)
Devil, Salt - (monster)
Devilbound Gnomish Prince - (monster)
Dinosaur, Mbielu - (monster)
Dinosaur, Ngobou - (monster)
Dinosaur, Spinosaurus - (monster)
Dinosaur Spinosaurus - Young - (monster)
Dipsa - (monster)
Dissimortuum - (monster)
Dogmole - (monster)
Dogmole Juggernaut - (monster)
Domovoi - (monster)
Doppelrat - (monster)
Dorreq - (monster)
Dragon, Cave - Adult - (monster)
Dragon, Cave - Wyrmling - (monster)
Dragon, Cave - Young - (monster)
Dragon Eel - (monster)
Dragon, Flame - Adult - (monster)
Dragon, Flame - Ancient - (monster)
Dragon, Flame - Wyrmling - (monster)
Dragon, Flame - Young - (monster)
Dragon, Mithral - Adult - (monster)
Dragon, Mithral - Ancient - (monster)
Dragon, Mithral - Young - (monster)
Dragon, Sea - Adult - (monster)
Dragon, Sea - Ancient - (monster)
Dragon, Sea - Wyrmling - (monster)
Dragon, Sea - Young - (monster)
Dragon, Void - Adult - (monster)
Dragon, Void - Ancient - (monster)
Dragon, Void - Wyrmling - (monster)
Dragon, Void - Young - (monster)
Dragon, Wind - Adult - (monster)
Dragon, Wind - Ancient - (monster)
Dragon, Wind - Wyrmling - (monster)
Dragon, Wind - Young - (monster)
Dragonleaf Tree - (monster)
Drake, Alehouse - (monster)
Drake, Ash - (monster)
Drake, Coral - (monster)
Drake, Crimson - (monster)
Drake, Deep - (monster)
Drake, Elder Shadow - (monster)
Drake, Paper - (monster)
Drake, Rust - (monster)
Drake, Star - (monster)
Drakon - (monster)
Dream Eater - (monster)
Drowned Maiden - (monster)
Dryad, Duskthorn - (monster)
Dullahan - (monster)
Dune Mimic - (monster)
Eala - (monster)
Eater of Dust - (monster)
Edimmu - (monster)
Eel Hound - (monster)
Einherjar - (monster)
Eleinomae - (monster)
Elemental Locus - (monster)
Emerald Eye - (monster)
Empty Cloak - (monster)
Eonic Drifter - (monster)
Erina Defender - (monster)
Erina Scrounger - (monster)
Far Darrig - (monster)
Fate Eater - (monster)
Fear Smith (Fiarsídhe) - (monster)
Fellforged - (monster)
Fext - (monster)
Feyward Tree - (monster)
Fiarsidhe - (monster)
Firebird - (monster)
Firegeist - (monster)
Flutterflesh - (monster)
Folk of Leng - (monster)
Forest Marauder - (monster)
Fraughashar - (monster)
Frostveil - (monster)
Garroter Crab - (monster)
Gbahali - (monster)
Gearforged Templar - (monster)
Genie, Al Aeshma - (monster)
Gerridae - (monster)
Ghoul, Beggar - (monster)
Ghoul, Bonepowder - (monster)
Ghoul, Darakhul - (monster)
Ghoul, Imperial - (monster)
Ghoul, Iron - (monster)
Giant, Desert - (monster)
Giant, Flab - (monster)
Giant, Jotun - (monster)
Giant, Thursir - (monster)
Glass Gator - (monster)
Gnarljak - (monster)
Gnoll, Havoc Runner - (monster)
Goat-Man - (monster)
Goblin, Dust - (monster)
Golem, Eye - (monster)
Golem, Hoard - (monster)
Golem, Salt - (monster)
Golem, Smaragdine - (monster)
Golem, Steam - (monster)
Gray Thirster - (monster)
Greater Death Butterfly Swarm - (monster)
Grim Jester - (monster)
Gug - (monster)
Hag, Blood - (monster)
Hag, Mirror - (monster)
Hag, Red - (monster)
Hag, Sand - (monster)
Harpy, Owl - (monster)
Harvester Devil - (monster)
Haugbui - (monster)
Herald of Blood - (monster)
Herald of Darkness - (monster)
Horakh - (monster)
Hound of The Night - (monster)
Hulking Whelp - (monster)
Hundun - (monster)
Ice Maiden - (monster)
Idolic Deity - (monster)
Imy Ut Ushabti - (monster)
Isonade - (monster)
Jaculus - (monster)
Kaguani - (monster)
Kaguani Greyfur - (monster)
Kalke - (monster)
Kikimora - (monster)
Kobold Alchemist - (monster)
Kobold Chieftain - (monster)
Kobold Trapsmith - (monster)
Kongamato - (monster)
Kot Bayun - (monster)
Krake Spawn - (monster)
Lantern Dragonette - (monster)
Lemurfolk Greyfur - (monster)
Lemurfolk (Kaguani) - (monster)
Leshy - (monster)
Library Automaton - (monster)
Lich Hound - (monster)
Likho - (monster)
Lindwurm - (monster)
Liosalfar - (monster)
Living Wick - (monster)
Lorelei - (monster)
Loxoda - (monster)
Mahoru - (monster)
Mallqui - (monster)
Malphas (Storm Crow) - (monster)
Mamura - (monster)
Mask Wight - (monster)
Mavka - (monster)
Mi-go - (monster)
Millitaur - (monster)
Mimic, Map - (monster)
Mindrot Thrall - (monster)
Mirager - (monster)
Miremal - (monster)
Mngwa - (monster)
Monolith Champion - (monster)
Monolith Footman - (monster)
Mordant Snare - (monster)
Morphoi - (monster)
Moss Lurker - (monster)
Mummy, Venomous - (monster)
Myconid, Deathcap - (monster)
Myling - (monster)
Naina - (monster)
Nichy - (monster)
Nightgarm - (monster)