When creating creatures, there are a couple of factors that you have to keep in mind, the greatest being 'what is its place in the food chain'. If your creature is meant to be the one at the bottom of the food chain then it isn't going to work if it struggles to reproduce. The 'rodents' need lots of numbers to make up for the loses to predators. Equally, if it is at the top of the food chain then what is the rival predator that helps keeps its numbers in check? Nature never comes up with a design that is utterly unbeatable. If there is a predator that has no natural enemies, other that it's own species, then you can almost guarantee that there will be a disease that culls out the numbers if it becomes too numerous. You just have to look at the fungi in the Amazon rain forest that reproduce by infecting insects with their spores and taking over their nervous systems. It sounds like something out of science fiction and it is disgusting but the more numerous the insect species the more likely it will be attack by its fungi predator.
You then need to consider that form follows function. For instance, my artwork seriously started with a sketch to explore how the fantasy creatures dragons could actually evolve.
First off, is the major issue of flight. That is the thing that sets dragons apart in many fantasy setting, the fact that they can fly. They also have a habit of lairing in cold, damp caves, despite the fact that caves are nearly totally unsuitable for a reptile. I put the two together and thought of hydrogen, the lightest of gases, the most flammable and the most readily available in the form of water, if you can crack it off the oxygen. The cracking process would produce a lot of heat, hence the dragons need for a cool lair; you do not want to over heat a body that is stuffed full of hydrogen, the result is a very big bang.
So hydrogen would proved the lighter than air lift needed for flight, would drive the dragon's need for coolness and a ready supply of water and also provide its ability to breath fire, which possibly evolved as a safety valve. Tubes from the gas bladders to the mouth, say opening under the tongue would pipe the hydrogen to where it could be safely mixed with oxygen, while a chemical gland along the lines of the bombardier beetle's would provide the spark needed to ignite the reaction.
Now for the illustrations out there, I have a little bug bear. Most books described the dragon's wing a bat-like and then the illustrators draw them with the 'little finger' of the wing limb folding along the forearm. That is physically impossible for a bat. A bat's wings is jointed exactly the same as the human hand and the human hand cannot touch the little finger to the outer edge of the forearm. Therefore, a dragon would more likely fold its wings by making fists and then pulling the 'arms' into the sides of its body.
As for how the dragon manages a six limb configuration I went with the idea that the spine extends beyond the arm shoulders into a second set of collar bones and shoulders to support the wings. The muscles of these second, wing arms would over lap the chest and back muscles of the arms and be anchored in them so that the dragon's chest would to all intents and purposes be double muscled, which would give the driving power to the wing beat.
I also decided that the arms would be shorter than the legs so the dragon would ride over its hips like a bipedal dinosaur, rather than be four legged animal on land. That would meet that the arms could support hands, much more useful for the grasping of prey and treasure, and the long tail would balance the weight of the body. I also decided that a dragon would most likely have spines on the end of the tail that could lay flat in flight or be opened as both an air break and a weapon as the tail would be the most vulnerable area, as I've never understood why the legends say that a dragons belly is unarmoured when a lizard has equally tough scales all over.
Going for all round maximum efficiency I designed the back feet to sport the sickle claws of a velociraptor, only on a much bigger scale. These would act like grappling hooks on cliff sides and a butchers hook when tackling prey of a similar or larger size than the dragon. If the claw structure was of the same design as a cats i.e. built up in multiply layers, then the risk of loosing a claw tip would be of no moment as the dragon would just have to pull off the damaged layers to reveal a new, sharp point.
There you are, that's my thought processes behind my dragon illustrations. If you are going to create fantasy creatures and 'monsters' in either your books or your art, my advice is 'the more outlandish they are, the more thought behind the function is needed'. Granted with artwork you have a little more leeway but in writing it is definitely true that you need to be able to justify all the spines and extra limbs your creature has. Have fun.
