RTF is a global modification that replaces all towns in the game.

Hmm... I heard that you can import creature models from Spore... I even saw instructions on how to do it somewhere, but when I tried, it didn't work for me (or rather, it worked, but only halfway – without textures). As an option, I could try experimenting with it again later and see if I can create something (hopefully, it will turn out well).