Creature Codex is the second 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, Creature Codex 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 Animal 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 NPC Codex in Appendix A is also excluded.
OGL Content:
Aatxe - (monster)
Acid Ant - (monster)
Agnibarra - (monster)
Ahu Nixta - (monster)
Ahuizotl - (monster)
Alabaster Tree - (monster)
Alkonost - (monster)
Alliumite - (monster)
Alp - (monster)
Ammut - (monster)
Ancient Mandriano - (monster)
Angel, Eye of the Gods - (monster)
Angel, Kinnara - (monster)
Angel, Ophanim - (monster)
Angel, Sammael - (monster)
Angel, Song - (monster)
Angel, Zoryas - (monster)
Ankou - (monster-include)
Ankou Soul Herald - (monster)
Ankou Soul Seeker - (monster)
Anophiloi - (monster)
Arborcyte - (monster)
Arcamag - (monster)
Arcanaphage - (monster)
Astral Snapper - (monster)
Avatar of Shoth - (monster)
Azeban - (monster)
Azi Dahaka - (monster)
Bar Brawl - (monster)
Barong - (monster)
Bat, Giant Albino - (monster)
Bat, Giant Vampire - (monster)
Bats - (monster-include)
Bearfolk Chieftain - (monster)
Bearmit Crab - (monster)
Bilwis - (monster)
Bloody Bones - (monster)
Bookkeeper - (monster)
Boot Grabber - (monster)
Carbuncle - (monster)
Cats of Ulthar - (monster)
Cauldronborn - (monster)
Centaur Chieftain - (monster)
Child of Yggdrasil - (monster)
Chuhaister - (monster)
Chupacabra - (monster)
Clockwork - (monster-include)
Clockwork Assassin - (monster)
Clockwork Servant - (monster)
Clockwork Soldier - (monster)
Corpse Thief - (monster)
Crimson Mist - (monster)
Cueyatl - (monster)
Cueyatl Moon Priest - (monster)
Cueyatl Sea Priest - (monster)
Cueyatl Warrior - (monster)
Dakini, Simhamukha - (monster)
Dark Eye - (monster)
Dark Father - (monster)
Dark Servant - (monster)
Dark Voice - (monster)
Demon, Alnaar - (monster)
Demon, Alpha Yek - (monster)
Demon, Cipactli - (monster)
Demon, Echo - (monster)
Demon, Fulad-Zereh - (monster)
Demon, Neophron - (monster)
Demon, Pishacha - (monster)
Demon, Plaresh - (monster)
Demon, Rattok - (monster)
Demon, Spree - (monster)
Demon, Vellso - (monster)
Demon, Wind - (monster)
Demon, Yek - (monster)
Derro, Speaker to the Darkness - (monster)
Derro, Witch Queen - (monster)
Desert Troll - (monster)
Devil Bough - (monster)
Devil, Fire Imp - (monster)
Devil, Infernal Knight - (monster)
Devil Shark - (monster)
Devil, Wind’s Harp - (monster)
Dhampir - (monster)
Dhampir Commander - (monster)
Dinosaur, Archaeopteryx - (monster)
Dinosaur, Lystrosaurus - (monster)
Dinosaur, Nodosaurus - (monster)
Doom Golem - (monster)
Dracotaur - (monster)
Dragon, Clockwork - One-Headed - (monster)
Dragon, Clockwork - Three-Headed - (monster)
Dragon, Light - Adult - (monster)
Dragon, Light - Ancient - (monster)
Dragon, Light - Wyrmling - (monster)
Dragon, Light - Young - (monster)
Dragon, Wasteland - Adult - (monster)
Dragon, Wasteland - Ancient - (monster)
Dragon, Wasteland - Wyrmling - (monster)
Dragon, Wasteland - Young - (monster)
Dragonborn, Edjet - (monster)
Dragonborn, Elementalist - (monster)
Dragonborn, Heavy Cavalry - (monster)
Dragonborn, Light Cavalry - (monster)
Dragonborn, Ouroban - (monster)
Drake, Bathhouse - (monster)
Drake, Fey - (monster)
Drake, Forest - (monster)
Drake, Moon - (monster)
Drake, Pact - (monster)
Drake, Peluda - (monster)
Drake, Skull - (monster)
Drake, Spider - (monster)
Dream Squire - (monster)
Dream Wraith - (monster)
Droth - (monster)
Dust Goblin Chieftain - (monster)
Dvarapala - (monster)
Ecstatic Bloom - (monster)
Elemental, Blood - (monster)
Elemental, Storm Lord - (monster)
Elemental, Venom - (monster)
Elf, Alchemist Archer - (monster)
Elf, Deathsworn - (monster)
Elf, Enchanter - (monster)
Elf, Servant of the Vine - (monster)
Elf, Shadow Fey Ambassador - (monster)
Elf, Shadow Fey Knight of the Road - (monster)
Elf, Shadow Fey Pattern Dancer - (monster)
Elf, Shadow Fey Poisoner - (monster)
Elophar - (monster)
Execrable Shrub - (monster)
Exploding Toad - (monster)
Far Wanderer - (monster)
Fear Liath - (monster)
Fierstjerren - (monster)
Flame Eater Swarm - (monster)
Flame-Scourged Scion - (monster)
Flesh Reaver - (monster)
Fleshpod Hornet - (monster)
Flying Polyp - (monster)
Foxin - (monster)
Fractal Golem - (monster)
Fragrite - (monster)
Fulminar - (monster)
Fungi, Wirbeln - (monster)
Gargoctopus - (monster)
Ghast of Leng - (monster)
Ghost Boar - (monster)
Ghost Boar, Elder - (monster)
Ghost Dragon - (monster)
Ghost Dwarf - (monster)
Ghoul, Darakhul High Priestess - (monster)
Ghoul, Darakhul Shadowmancer - (monster)
Ghoul, Necrophage Ghast - (monster)
Ghoul, Tar - (monster)
Ghoulsteed - (monster)
Giant, Blood - (monster)
Giant, Cacus - (monster)
Giant, Cave - (monster)
Giant, Haunted - (monster)
Giant, Laestrigonian - (monster)
Giant Moth - (monster)
Giant Moth, Rimewing - (monster)
Giant Moth, Shockwing - (monster)
Giant Moth, Sootwing - (monster)
Giant, Mountain - (monster)
Giant, Void - (monster)
Gloomflower - (monster)
Gnoll Slaver - (monster)
Gnomish Knife Cultist - (monster)
Goblin, Chaos-Spawn - (monster)
Golem, Alchemical - (monster)
Golem, Altar Flame - (monster)
Golem, Armory - (monster)
Golem, Bone - (monster)
Golem, Bronze - (monster)
Golem, Lesser - Glass - (monster)
Golem, Lesser - Hair - (monster)
Golem, Lesser - Mud - (monster)
Golem, Lesser - Wood - (monster)
Golem, Lotus - (monster)
Golem, Manastorm - (monster)
Golem, Paper - (monster)
Golem, Paper - Swarm - (monster)
Golem, War Machine - (monster)
Goliath Longlegs - (monster)
Goreling - (monster)
Grave Behemoth - (monster)
Greater Rakshasa - (monster)
Greater Scrag - (monster)
Green Abyss Orc - (monster)
Green Knight of the Woods - (monster)
Grindylow - (monster)
Gugalanna - (monster)
Gulon - (monster)
Gumienniki - (monster)
Hallowed Reed - (monster)
Horned Serpent - (monster)
Hound of Tindalos - (monster)
Hungry Ghost, Gaki - (monster)
Hungry Ghost, Preta - (monster)
Ichneumon - (monster)
Ijiraq - (monster)
Incinis - (monster)
Inkling - (monster)
Iron Sphere - (monster)
Jaanavar Jal - (monster)
Jiangshi - (monster)
Jinmenju - (monster)
Kallikantzaros - (monster)
Kappa - (monster)
Karakura - (monster)
Keg Golem - (monster)
Kitsune - (monster)
Kobold, Elite - (monster)
Kobold, Junk Shaman - (monster)
Kobold, King - (monster)
Kobold, Swolbold - (monster)
Kobold, Wizard - (monster)
Korrigan - (monster)
Kryt - (monster)
Külmking - (monster)
Kuunganisha - (monster)
Lady In White - (monster)
Lamassu - (monster)
Lamia, Serpentine - (monster)
Lamia, Serpentine - Matriarch - (monster)
Leonino - (monster)
Lich, Hierophant - (monster)
Lich, Pact - (monster)
Living Shade - (monster)
Living Star - (monster)
Lou Carcolh - (monster)
Lycanthrope, Werebat - (monster)
Lycanthrope, Werehyena - (monster)
Mandrake - (monster)
Mandrake, Great - (monster)
Mandriano - (monster)
Megapede - (monster)
Minotaur, Labyrinth Keeper - (monster)
Minotaur, Lost - (monster)
Minotaur, Moon Priestess - (monster-include)
Moon Nymph - (monster)
Moonchild Naga - (monster)
Morko - (monster)
Mytholabe - (monster)
Nachzehrer - (monster)
Nalusa Falaya - (monster)
Necrotic Tick - (monster)
Nian - (monster)