Find errorlog.txt in your DF folder. Find the creature entries it's complaining about. Delete them.
Thanks for the pointer! Errorlog is complaining about a lot of creatures and all six entities as being duplicated
Duplicate Object: creature BIRD_ROC
Duplicate Object: creature SEA_MONSTER
Duplicate Object: creature SEA_SERPENT
Duplicate Object: creature MERPERSON
Duplicate Object: creature HYDRA
Duplicate Object: creature HARPY
Duplicate Object: creature NIGHTWING
Duplicate Object: creature STRANGLER
Duplicate Object: creature BLENDEC_FOUL
Duplicate Object: creature WEREWOLF
Duplicate Object: creature GRIMELING
Duplicate Object: creature BEAK_DOG
Duplicate Object: creature PIXIE
Duplicate Object: creature FAIRY
Duplicate Object: creature WOLF_ICE
Duplicate Object: creature BLIZZARD_MAN
Duplicate Object: creature SASQUATCH
Duplicate Object: creature YETI
Duplicate Object: creature MINOTAUR
Duplicate Object: creature ETTIN
Duplicate Object: creature CYCLOPS
Duplicate Object: creature GIANT
Duplicate Object: creature COLOSSUS_BRONZE
Duplicate Object: creature SATYR
Duplicate Object: creature DRAGON
Duplicate Object: creature UNICORN
Duplicate Object: creature OGRE
Duplicate Object: creature TROLL
Duplicate Object: creature GREMLIN
Duplicate Object: creature KOBOLD
Duplicate Object: creature GOBLIN
Duplicate Object: creature ELF
Duplicate Object: creature HUMAN
Duplicate Object: creature DWARF
Duplicate Object: entity SUBTERRANEAN_ANIMAL_PEOPLES
Duplicate Object: entity SKULKING
Duplicate Object: entity EVIL
Duplicate Object: entity PLAINS
Duplicate Object: entity FOREST
Duplicate Object: entity MOUNTAIN
Duplicate Object: creature BIRD_ROC
Duplicate Object: creature SEA_MONSTER
Duplicate Object: creature SEA_SERPENT
Duplicate Object: creature MERPERSON
Duplicate Object: creature HYDRA
Duplicate Object: creature HARPY
Duplicate Object: creature NIGHTWING
Duplicate Object: creature STRANGLER
Duplicate Object: creature BLENDEC_FOUL
Duplicate Object: creature WEREWOLF
Duplicate Object: creature GRIMELING
Duplicate Object: creature BEAK_DOG
Duplicate Object: creature PIXIE
Duplicate Object: creature FAIRY
Duplicate Object: creature WOLF_ICE
Duplicate Object: creature BLIZZARD_MAN
Duplicate Object: creature SASQUATCH
Duplicate Object: creature YETI
Duplicate Object: creature MINOTAUR
Duplicate Object: creature ETTIN
Duplicate Object: creature CYCLOPS
Duplicate Object: creature GIANT
Duplicate Object: creature COLOSSUS_BRONZE
Duplicate Object: creature SATYR
Duplicate Object: creature DRAGON
Duplicate Object: creature UNICORN
Duplicate Object: creature OGRE
Duplicate Object: creature TROLL
Duplicate Object: creature GREMLIN
Duplicate Object: creature KOBOLD
Duplicate Object: creature GOBLIN
Duplicate Object: creature ELF
Duplicate Object: creature HUMAN
Duplicate Object: creature DWARF
Duplicate Object: entity SUBTERRANEAN_ANIMAL_PEOPLES
Duplicate Object: entity SKULKING
Duplicate Object: entity EVIL
Duplicate Object: entity PLAINS
Duplicate Object: entity FOREST
Duplicate Object: entity MOUNTAIN
So I re-checked the raws, and couldn't find what I did wrong - then it dawned on me. I made a backup copy of creature_standard and entity_default before I changed anything. DF was reading both my modded version and the backups, causing the duplication. After moving my backups into a zip the next world genned normally.
What?! Am I the only one who thinks this is awesome? Leave them be and set up the first ever DF civilisation of scally things
I just may try that, I also uploaded the save if you want to try your hand as cave crocs striking the earth, with giant olms, toads, and bats as potential raiders.