I've come here to seek help because i've encountered something strange, i am not sure whether this is a genuine bug or somehow a recursive act of my modding, error log isn't spitting anything out at me and the remainder of the game seems stable and concise.
When you create custom creature fields and apply them restrictively, it appears to enable a strange behaviour where all creatures that are "general familiarity" either through specific tags or [USE_UNDERGROUND_CREATURES] etc entity tags eventually become domesticated pets, this includes all of the 1st and limited 2nd layer creatures described by use_underground creatures. Including all creatures in the surrounding biomes.
In the given picture, i was able to embark with giant crocodiles, and i will await to see if i can order more crocodiles or underground creatures from merchants but this is extremely wierd and appears to work through the list taming them in world-generation. As you can see jabberers are still untamed but much of everything else is in 20 years of world generation.
In my mind its a bugfix to something that hasn't been a issue in a while but was originally badly implemented when DF moved over to the new animals taming system from dungeon masters a long time ago.I applied this code onto the dwarven & human civilisations as so they wouldn't tame my evil & good pets with the class by accident and are locked out from even doing so accidentally in world-generation.
[ANIMAL]
[ANIMAL_FORBIDDEN_CLASS:EVIL_PET]
[ANIMAL_FORBIDDEN_CLASS:GOOD_PET]
which were custom creature classes i made to keep my raws organised and work with [ANIMAL], none of the listed animals cross into that list and other civilisations with the creatures allowed seem to follow the [ANIMAL] rules with no particular complications. However, creatures with [CREATURE_CLASS] allowed through [ANIMAL] have unlimited range for how deep underground they are taken looking for creatures to tame with that class tag.
Leading to examples of blind cave trolls (layer 2 usually inaccessible) being tamed in world generation by goblins and wreaking havoc rather than the typical first layer response. Which was what i wanted from the tag but suprised me to actually recieve it, thinking that i would have had to make a goblin fortress first and tame them myself.
To end, i don't know what caused this precisely from a game logic standpoint, but it should hopefully serve as instructive to you in avoiding it if you want to, i've reconstructed this effect on multiple worlds using virtually the same entity and creature raw build.