Shouldn't game design come before sprites? Define game balance, how many monster of each kind etc. So we know what we have to draw instead of making random stuff which might not be useful? We don't even know if monster are going to evolve!
I agree with the 5 Chinese elements. For each element we could have the same number of monsters. we could have 3-4 common monsters, 2 rare monsters and 1 really rare (and really powerful) monster.
I wonder if we can really do game design like that on the forum, with people throwing random ideas. Maybe someone should suggest a defined and complete game design, then we discuss it, and then we start a challenge to draw sprites.
This post may sound a bit negative, but I just need this challenge needs to be a little more structured. Of course you can continue to draw nice monster as you already did