Almost definitely that or immigrants, that's what does it for me.
On a side note, one world had truly epic dark elves bringing me cave crocodiles and virtually every other underground exotic pet other than the GCS. Excited I genned another world-and noticed something. Despite being in every way superior to normal elves (including by having steel weapons/armor for trade), they seem quite liable to get wiped out in world gen, usually quite early on. Sometimes it seems they get wiped out by the normal elves. Is this intentional?
By the way, I'm not sure, but I think the dragon issue may be caused by having too many parts, as not all of them appear to be able to show up on the body parts list in-game. On another note, all giant shadow spiders show up as males-and all GCS appear to be female for some reason. (Might be random gen, but seems like it to me) modded the game to tame the bane dragon i caught, and it had the red + flashing on it's tile to indicate a serious injury-possibly a missing organ somewhere.
One more, very minor thing: (freshwater)eels are actually able to survive on land for periods of time, and at least one kind does this as it migrates from rivers to oceans, so they can also move over the ground-they aren't 100% restricted to the water. I suggest taking the giant eel's water dependency away and just leaving it with swims innate.
Edit: And you did a damn good job of making evil regions that much more terrifying! Nothing like seeing 10 flying implords, more than ten cave trolls, and heaven knows how many other nasties i could SEE to give a player pause.