Good question to ask myself... and thereby the community, actually.
The way I see it, wildlife in DF (or any other fantasy realm) can be divided into 3 types:
Beasts - This is your basic critter. Bears, Wolves, basically anything dangerous that is nonetheless Natural and Mundane. Humans fit into this category, as well. Obviously these should be the most numerous critters, and the easiest to kill.
- Lions
- Tigers
- Bears (oh my)
- Man (because he is the most dangerous animal)
- Dwarves (because they're crazy but not impossible)
Fantastical - In DF-terms, this is a semimegabeast. Something about this critter is just not natural. It should not exist, for some reason or another, but nonetheless it is a mortal being. Elves fit this in most realms. For society as we know it to truly blossom, there cannot be an overwhelming number of Fantastical beings.
- Elves (as I see them, they live for a millenia... not natural but still mortal)
- Wyvrens (They existed, as Quetzalcoatls, Pterodactyls, etc)
- Chimera (Add a bunch of Beasts into one body, and there you go)
Megabeast - Big. Nasty. Monstrous. These creatures defy the very logic of what it means to be a Natural critter. They do not die from old age. And if they are not destroyed, civilization cannot move forward. Who can run a kingdom when a Dragon will come by and knock everything down. These are the things that Heroes fight.
- The Dragon
- The Collosus
- The Terrasque
- Anything starting in "The"
So basically, when the smoke of worldgen clears, I'd like to have a few Megabeasts, two scoops of Fantasticals, and a good number of Beasts.