Yes, I've still got WotC's changes to humanoid creatures on the mind.
And no, this is definitely not a novel concept, I even considered doing it years ago just as a way to make a game that felt more mythological and less like Tolkien or his many fantasy fiction descendants (Wheel of Time, Shanarra, etc).
I'd like to one day run a campaign with no humanoid monsters. No goblinoids, orcs, gnolls, lizard men, etc. Just get rid of all of them. No demi-humans, either. Let's get rid of the bandits, pirates, brigands, etc. while we're at it.
All the PCs would be human. All the monsters would be normal or giant animals, slimes and oozes, or non-tribal monsters. There would still be intelligent monsters, but any intelligent monsters encountered would not be organized bands larger than family units.
It would be a different feel for a campaign, and would make some classes (my homebrew Berserker [Barbarian] class, the AD&D Ranger) less than optimal. So maybe stick to the BX/BECMI classes.
Looking at Basic (Mentzer) that would leave us with:
Giant Ant, White Ape, Rock Baboon, Bats -normal, giant/vampire, Bears - black, grizzly, polar, cave, Giant Bee, Giant Beetle - fire, tiger, oil, Boar, Carrion Crawler, Great Cat - mountain lion, panther, lion, tiger, sabre-tooth tiger, Giant Centipede, Doppleganger, Dragon - white, black, green, blue, red, gold, Giant Ferret, Gargoyle, Gelatinous Cube, Ghoul, Gray Ooze, Green Slime, Harpy, Living Statue - crystal, iron, rock, Giant Lizard - gecko, draco, horned chameleon, tuatara, Giant Locust, [Lycanthropes -- keep them in or not? Hmm...], Medusa, Minotaur, Mule, Ochre Jelly, [Ogre -- again, keep or not?], Owl Bear, Rat - normal, giant, Robber Fly, Rust Monster, Shadow, Giant Shrew, Shrieker, Skeleton, Snake - spitting cobra, giant racer, pit viper, sea snake, giant rattler, rock python, Giant Spider - crab, black widow, tarantella, Stirge, Wight, Yellow Mold, Wolf - normal, dire, Zombie.
That's a fair amount. Adding in Expert, we get:
Animal Herd, Basilisk, Black Pudding, Blink Dog, Caecillia, Camel, Chimera, Cockatrice, Giant Crab, Crocodile - normal, large, giant, Cyclops, [Devil Swine if Lycanthropes are in], Displacer Beast, Djinni, Dryad, Efreeti, Elemental, Elephant - normal, prehistoric, Giant Fish - bass, rockfish, sturgeon, Giant - hill, stone, frost, fire, cloud, storm, Golem - wood, bone, amber, bronze, Gorgon, Griffon, Hellhound, Hippogriff, Horse - riding, war, draft, Hydra, Insect Swarm, Invisible Stalker, Giant Leech, Manticore, Mummy, Nixie, Pegasus, Pterodactyl - normal, pteranodon, Purple Worm, Rhagodessa, Roc - small, large, giant, Salamander - flame, frost, Giant Scorpion, Spectre, Water Termite - swamp, fresh water, salt water, Giant Toad, Treant, Triceratops, Troll, Tyrannosaurus Rex, Unicorn, Vampire, Wraith, Wyvern.
That's a good amount of monsters. And of course, there are the Companion/Masters sets, and AD&D Monster Manual to get more from. Easily possible to run a campaign this way.