Dinos and Aztecs
Heya.
For what it's worth, I came across Sons of Azca and The Milenian Empire, OD&D Hollow World accessories, and they have a few original monsters.
Sons of Azca:
Archelon (giant prehistoric sea turtle)
Baluchitherium (giant prehistoric rhino; appears in MMII as "indricothere")
Gruquotec (savage humanoids who share common ancestors with orcs and goblins)
Feathered Serpent (like a mini-couatl)
Hyenodon (already in the CC)
Lycanthrope, Werejaguar (appears in the Ravenloft 3E monster book)
Moan Bird (spooky phase-shifting raptor/owl)
Nothosaurus (voracious seal-like dino-kin, related to plesiosaurus)
Tanystropheus (prehistoric lizard with extremely long neck)
Tepictoton (Aztec brownie-like fey)
Tzitzimitl (demonic servants of Death)
The Milenian Empire:
Agrisian Jellyfish (giant glowing enveloping jellyfish that come in swarms)
Amorian Hound (primaeval canines with large fangs and both fur and scales)
Shaggy Auroch (furry prehistoric oxen)
Zargosian Bat (huge bats that spit venom)
Zargosian Changeling (similar to a doppelganger, but a few differences in their powers and appearance)
And also Nightwail, a Hollow World adventure:
Discorporate (mental projections of humans -- not undead, but similar in appearance -- with the power to teleport matter)
Giant Annelid (gigantic worms that once threatened the Hollow World)
Gridbug (large dragonfly-like insects without wings)