Mechanics vs. Flavor text

[Hussar]

As has been said, the question is backwards. You develop new mechanics as needed to represent elements in your world. Making up new mechanics to represent nothing in particular would be absurd. What credible world was ever designed around a collage of rules?

Actually, I would also disagree with this. Take the Binders from Tome of Magic. Here is something which does not exist in any world currently - it's a completely new mechanic. The idea that you draw power from these entities to enhance your character is, as far as I know, completely new. Granted, the idea of the fluff may have been around for a while, although in a very different form, but, from the looks of it, the mechanic came first and the fluff was then designed to explain the mechanic.

I'm sure there are others as well. It has been mentioned that certain monsters are created, not because they model some mythical creature, but because there is a perceived need/want for a CR 5 creature that swallows whole. (I'm not sure if such a creature exists, but the Design and Development column at WOTC suggests that monster development follows this sometimes.)

I would agree that usually, we design mechanics to model some sort of reality in a given setting, but, sometimes, the mechanics come first and the reality follows after.