PTW. I love this idea. It would make modding very hard, though.
Hmm. Yes, the process would have to change a little. I think you'd want to keep the current ability to create specific creatures, but then also mod the base creatures. For example, let's say you want to add chocobos to DF. It has feathers, a beak, somewhere between horse and ostrich in size, etc. Then when you generate a world, it populates with various kinds of chocobo, Grey Chocobo, Minkot's Chocobo, Mountain Chocobo, Murk of Ticks Chocobo, etc. Perhaps somewhere in there, you can specify that some species of chocobo can fly, while others cannot. There could be a %age range, like there are for attributes and personalities, although I think it would be simpler to just mod in "chocobos" and "flying chocobos".
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But I also wouldn't want to find that I'm forced to analyse each individual within the fauna as I would a Forgotten Beast , where previously there's a very useful shortcut that even within this alien dwarf-inhabited land I can very quickly identify the relevance of a critter to my endeavours.
("What's that creature that just wandered onto my map? Is it going to be troublesome? Will it try to half-hinch food or valuable goods? Is it likely to predate my dwarves? Do I have to watch out for it being able to swim or fly? I think I need to check its description very carefully..." vs "Ah, a kangaroo... Have I sufficient meat? I could safely ignore it or re-activate my Hunter's skills anyway...")
Yeah, I think it's important to strike the proper balance between amusement and usability. But this being Dwarf Fortress, usability is not necessarily at the forefront.
I do think I would want a clear list of features, like in the DF wiki, things like Adult Size, Diet, Mount/Pack/Pet/Trainable, Air Breather/Water Breather/Amphibious, Flying Y/N, quick list of body parts. In the spirit of NetHack and Pokedex, maybe information comes to you slowly as you encounter each animal. For example, the animals that live around the mountains and grasslands of your starting civ are fully described, but if you embark to a far-off jungle, you won't see any information about the new animals until you've caught or dissected a couple. If you encounter the equivalent of a Kea, you may not see "Steals Stuff" in its description until after it has stolen something. If an animal can be trained for war, you won't know until your animal trainers have tried to tame one. Things that you can see with your eyes should be given for free. Or since taverns and books will come first, maybe you can hear about animals from travelers or trade an elf for a zoological manual.