Martin Snyder of Ethermoon has talked about the process of designing units for their real time strategy game, Strifeshadow -
"We start with a functional idea of what a unit, or a group of units is, and work from there to create a believable and interesting mythos. Take our Chaos Elemental for instance. The initial idea for this unit was a "fast, flying summoned skirmisher best used for air-to-air attacks" - not too interesting by itself. From this building block, we determined what exactly this unit .. would look and feel like in our fantasy universe."