Once we settled on the idea of the dragons being themed on elements, with primary dragons of "pure" elements breeding together to create hybrid dragons that mixed those elements, and favored one or the other, we had to figure out what those would be. The traditional Western 4 elements (Earth, Water, Air, and Fire) were not enough. We could add Ether for a 5th, or the Eastern Metal and Wood. In true Backflip Studios fashion, the Art Director came in on a Monday morning with this list of 7 from our longer list of possibilities. In the end, because we had opposing elements that could not breed together initially, the odd number was a bit awkward. Fixed later when we added Metal as a primary.
These were designed to run a gamut of emotional responses, and to use different body patterns so that the hybrids could mix and match, and we would have variety. We were limited to 12 bones initially! The babies used 3.