There are lots of things you can do to make dragons "work," but most, if not all, of them would make them pretty ineffectual as "dragons" in the D&D and fantasy context. If you want your dragons to be blimps, then technically you could make them fly. The pterosaur, as I mentioned is about 1/3 the weight of the giraffe and about 1/3 the durability of the giraffe as well. The idea of a "real" dragon picking a fight with anything (and being able to fly) is fairly laughable. Similarly, the idea of a six-limb creature is impossible from our world, but such biology could spring up somewhere else. That is no more unlikely, than gas-filled bones and sacs for breathing fire and flight really.