I was peacefully modding DF when I found some Forgotten Beasts civ in neighbors.
To tell the truth that was not quite exactly what I was trying to get but in some way awesome too. I checked them through legends:
They are acting like civilisated creatures even being enormous skinless vipers. I do really like DF for things like these. And I think it's not a bug but a feature. Or even special rare hardcoded event. I fondly sniffed all my raws for they entity description. And they continued to appear in every next wordgen with suspicious human civ missing. Finally I've found the reason. Creature HUMAN was declared twice in two different files.
So the mysterious forgotten beasts race is the human race that due to creature declaration mistake changed their inhabitants to terrifying monsters with civilized habbits. That's really a Holly Grail. I spend some time trying to mod raws to get both FB and human race back all together. (I'm using vanilla DF for modding) And learned that things are not so easy I thought before. It looks like the resulting civ creature is totally random even if I only change name of raw file. Example:
(Suspense music!)
At last I managed to get it work correctly.
But I'm not sure it's going to show the same result on an another computer or even after my computer reboot.
Anyway here are mod files.
http://dfiles.ru/files/uft8u06xgAnd saved world
http://dfiles.ru/files/7poe0dzt8I don't know yet are they going to siege my civ and how it looks. Hope the game won't crash or anything like that.