The Monster Manual already has them listed alphabetically and by level. Here is a list of everything in the monster manuel, with my speculations as to their original homes before the formation of the world.
The World
- Basilisk - Possibly Elemental Chaos
- Battlebriar
- Bear
- Beetle - Possibly Elemental Chaos
- Behemoth - Possibly Elemental Chaos
- Boar, Dire
- Bodak - Created from humanoids in this world
- Boneclaw - Created by ritual in this world
- Cambion - Part comes from the Nine Hells, part from The World
- Carrion Crawler - probably the result of influence from the Far Realms, but grown here
- Chimera - This has got to be the result of bizarre genetic experimentation
- Colossus
- Death Knight
- Dracolich
- Dragon - The epitome of the creations of this world, their natures are rooted in the Elemental Chaos, but their minds reflect the Astral Sea
- Dragonborn - I speculate that they are an ancient, stable line of half-dragons, an ancient branch of humankind
- Dragonspawn
- Drake
- Eidolon
- Ettercap
- Flameskull
- Foulspawn - Far Realm inspired, locally grown
- Gnolls - I have an idea that they were created as shock troops in some ancient conflict by an evil mage, probably named Yeenoghu, and continue to revere him as a god to this day, having long outlived their usefulness.
- Golem
- Guardian
- Halfling - One day, Melora and Sehanine decided to create an "improved" version of the human race, and halflings were the result. The idea was to moderate the relentless urbanization occuring at the time.
- Helmed Horror - Materials produced locally, motive power brought from Elemental Chaos.
- Homunculus
- Hook Horror - I feel like something like this has to have come from somewhere else, but where is unknown.
- Horse - I'm tempted to say the Celestial Charger is actually the original version, and the regular horse is adapted to this world.
- Hound - A natural beast that found a place everywhere within one step from the world
- Human - The ideal I base this on is that humans are the only beings native to this world, though in many cases, they are newer to this world than the beings that came when the world was first forming.
- Hyena
- Kobold - Created long ago as servants to a particularly powerful dragon
- Kruthik
- Larva Mage
- Lich
- Lizardfolk
- Lycanthrope - I'm thinking of going with an Elder Scrolls type of lore here, making various kinds of lycanthropes the result of (long ago) primordial or demonic perversions of existing beings.
- Minotaur - "Animal people" in general are the result of either blessings or curses by gods, genetic experimentation, or perhaps they are both the same thing, lost to the mists of time. Ooh! Shades of Sitchin. The animal people were experiments from before Man was made? Man shares the blood of the Gods?
- Mummy - Are you my mummy?
- Panther - Not really sure where they're originally from, but they seem to inhabit both the Feywild and the Shadowfell
- Rakshasa - Another animal being, this time with explicitly stated potential demonic heritage. Starting to think all the animal creatures were created as servents to Primordials and Demons (except Kobods, which served a certain dragon king), while others were created by the gods.
- Rat
- Sahuagin
- Shambling Mound
- Shifter - Proof that lycanthropes are humankin, as they can breed with humans...
- Skeleton
- Skull Lord
- Snake - Not sure, but there are also EC and Shadowfell equivalents. May be originally from Tytherion, though.
- Spider
- Tiefling
- Troglodite
- Vampire - Orcus' equivalent of a lycanthrope?
- Vine Horror
- Wight
- Wolf
- Worg
- Zombie
- Banshrae
- Boar, Thunderfury
- Choker
- Crocodile - These seem natural enough, but there's a fey equivalent
- Cyclops
- Displacer Beast
- Drow
- Dryad
- Eladrin
- Gnome
- Hag
- Harpy
- Lamia
- Owlbear
- Quickling
- Satyr
- Treant
- Unicorn
- Bat, Shadowhunter
- Berbalang - Nothing in the description specifies this, but given their diet of the memories of deceased sentient beings and their association with undead, this seems right
- Dark One
- Devourer
- Ghost
- Nightmare
- Nightwalker
- Shadar-Kai
- Sorrowsworn
- Specter
- Umber Hulk - There's a shadow equivalent, and they don't "feel" natural.
- Wraith
- Wyvern - Perhaps originally a Shadowfell reflection of local drakes?
- Angels
- Marut
- Naga - Primordial Nagas don't seem to fit in this category. I shall have to give these more thought
- Sphinx
Tytherion
- Yuan Ti - These were definitely descended from the "people" of Tytherion
Celesta
Chernogger
- Goblin - I'm thinking that Goblins originated as a war party sent from Chernogger by Bane to conquer this world. They were, to a large measure, successful. The hobgoblins that came created goblins and bugears to serve as scouts and shock troops, respectively. Their time in this world and their subsequent separation from their kin in Chernogger has reduced them significantly from what they once were.
- Ogre
- Oni
- Orc - Ogres, Onis, and Orcs were troops sent from the field outside Chernogger to counter Bane's move on this world. The Goblin Age was a truly dark time, with everybody either ruled by hobgoblins or terrorized by orcs and ogres. (Note: what's their relation to the giants?
Nine Hells
- Devil
- Scorpion
- Archons
- Azers - Not sure if they were a part of the world's creation enslaved by the giants during its creation, or if they originate in the Elemental Chaos
- Bat, Fire
- Bulette - Natural beasts, but they feel like something originally from the Elemental Chaos
- Dwarf - I've decided that the ancient ancestors of the Dwarves came from the Elemental Chaos, and were slaves of Giants and Titans. Those giants and titans that came to fashion The World brought Dwarf slaves with them, and it was these that rebelled and became the Dwarves we know today. Moradin may not have been the creator of all dwarves, but rather the god that nudged the dwarves of this world to freedom. He didn't create the Azers or the Galeb Duhr, but he did "create" the dwarves.
- Efreet
- Elemental
- Ettin
- Fomorian
- Galeb Duhr
- Gargoyle
- Giant
- Gorgon
- Griffon
- Hydra
- Magma Beast
- Manticore - Not explicitly stated, but these beings seem like they'd be more at home in the Elemental Chaos than the world... though they could also be weapon beasts bred by some long lost empire.
- Purple Worm - This has got to be originally from the EC.
- Roc
- Roper - Wasn't expecting it, but they're clearly marked "elemental"
- Salamander
- Slaad
- Stirge - Basically giant mosquitos, they are merely parasitic on their home plane.
- Troll - These guys seem adapted for a place that is extremely dangerous and constantly changing... plus they speak giant.
- Abominations
- Demon
- Ghoul
- Orcus
- Rot Harbinger
- Aboleths
- Balhannoth
- Beholder
- Chuul
- Destrachan
- Gibbering Beast
- Githyanki
- Githzerai
- Grell
- Grick
- Kuo-Toa - The madness of Kuo-Toa is their reaction to geometry and physical laws that differs subtly from their homeworld.
- Medusa - Not explicitly stated, but they feel like a powerful alien race that might be found in small number attempting to build power on any world.
- Mind Flayer - Duh. Once had an empire spanning the stars, before the Gods rose.
- Swordwing
- Warforged - Not precisely Far Realm, but thew few that exist in this world (if any) come from another world entirely, having been brought across the Farm Realm for one reason or another.
- Doppelganger - I really don't know where a creature like this can be considered to have come from. They could be a changed line of human. They could have originated as tricksters from the Feywild. They could be from anywhere, and seem to have no natural existence outside their insinuation into other societies.
- Grimlock - Not sure whether these are another being from the Far Realm, perhaps from another world altogether (like the Gith) that hitched a ride here, or a variety of human that was long trapped in the Underdark, or perhaps even something from the Feywild. May not matter.
- Ooze - No idea.
- Otyugh - Tentacled horror from the far room? Or land squid? Or perhaps both?
