Kind of on this subject: It would be cool if you set things up so that modders can easily make their own creatures. Maybe they could come as "creature files" or something, so you just have to place the file into the right folder, and presto! The game will spawn that creature.
When there are creatures, modders will definitely be able to make their own. In fact, as currently designed, that's how I'll be adding them to the core game anyway... as a built-in mod.