Haha, if you're new to the whole modding business go slow first
Try and add, say, turkeys first.
When you succeed, move on to a creature requiring new tags in the body_default.txt, like... Oh, I know! There's this creature called Snøvsen from a danish childs novel, he were depicted in films even :p Anyways, there's a youtube link to some scenes with him, just ignore the music :p http://www.youtube.com/watch?v=X-kfi4r66K0
After that, try adding your own/existing creature as a civ.
All I'm trying to say, is that you just don't jump in it and succesfully add new creatures, especially if you wan't them to be unique...
Better to do it now I suppose than in the next version, modding are going to be hardcore!
Ive added some animals and plants already. I have pigs, domestic camels, domestic alligators, grapes and potatoes amoung the many weapons I added.
Im pretty confident with my modding abilities, though Ive never made a working civ before.
Seems like you know how to do a creature then
A civ isn't as hard as a creature using custom body parts, the ethics are a bit sketchy though, Toady posted in the future fortress thread how they worked, don't have the link since that thread have over 8000 replies, but basically, if your ethic regarding, say, torturing prisoners, were okay if justified, you'd gain 2+ point with other civs having the same ethic, 1+ with the ones having nearly the same, can't remember which one it were for all the different, but something like okay in self defense, 1- if it's over on the other end (impartial, maybe..) and -2 if it's the entirely opposite.
So, if you understood that garbage, a civ can have between -20 and 20 "ethic points" with another civ, the closer to -20 the more likely they are to go to war, the closer to 20 the more likely they are to be at peace, since alliances aren't in the game right now.
Toady posted the entire table regarding this, but I can't find it..
But yeah, that's how it works.