Let us review the external appearance of the Chinese dragon: thin, long body with four legs, over sized head, and no wings. Flying is out of the question. There have been no creatures proven in science that could fly without wings or wing-like structures. The legs and talons are also out of proportion, but, if enlarged, can make a plausible serpent. Pretty much, the Chinese and Asian dragons can exist, but not as we believe them to. They would be a little more like snakes with legs.
All right, people have argued this: Chinese dragons could be filled with a gas that makes them "float", and the arms and legs were just for steering. This is highly implausible if you really consider it. A gas within a biological being that is enough to make it float; yet it still have the organs to live? The internal body would have to be huge to do that! So, again, flying is out of the question.
The other problem is the body shape and form. Should there be four legs, then the body would have to be thicker near the legs, at least, because the legs need certain muscles to work. Then, after this, you would have a creature the looks like a Cheetah in scales. Is this possible? Not really. Lizards are shaped differently then mammals. The outer appearance is definitely a snake-like reference, so, in that regard, these dragons might well be real. They would appear very similar to serpents.
A Western Dragon is a huge reptile with four legs, a long tail, with two wings. Its head is that of an alligator or a crocodile (wedge shape, actually) and is usually accompanied with horns, spines, spikes, and a massive amount of other things. I have a strange feeling that wings would be out of the question; I have yet to see a non-insectoid or non-aquatic creature with more than four limbs; four feet, two wings? Have you seen a bird with arms? I have not. What about a huge insect? I have not seen one, either. So, the idea of the Western style dragons being real is questionable.
Then there is the crocodile and alligator shaped head styles. This actually makes them possible! A creature most certainly can evolve. Scientist believe that the first forms of life came out of the water, rapidly evolved, and then became other creatures. Now the real question is: Could a creature evolve enough to actually grow two extra limbs? And, not so much as that they are extra limbs, but that they are wings!
Another idea is that Western dragons is that they evolved from dinosaurs like the Pteronadon. The idea is far more likely than that of the alligator or crocodile. Why? Well, evolution is the betterment of a species, so if the Pteronadon were to evolve, it could be a step toward draconic figures. The wing muscles would have to develop, though, because the Pteronadon did not have wings like the ones we consider to be draconic. Also, they would have to grow arms. But, if a creature can go from walking on mainly all four and evolve into a creature that stands on two legs, there might be another chance of the creature adding a few limbs, especially non-complex ones like arms. When I mean arms, I mean frontal legs, too! There could have also been a mutation.
However, if the Pteronadon did evolve, it wouldn't have grown too much larger. The Pteronadon was perhaps the size of a turkey, if that, although some larger ones have been found, and that is certainly not a Western dragon. The other problem is that dragons are considered cold blooded. You see, they must have been warm blooded to have flown in cold conditions, or they would need to fly only when the sun was out.
So, it might be possible that Western dragons could be real. However, they would be much smaller then we think they are, and they would have to be warm blooded. That is another mystery entirely.
Sea Dragons are the most likely to really exist. The human race has yet to discovery everything about marine biology, and the oceans are so deep and vast that we still need to develop technology to find everything. Now, eels can grow quite large, and so can other kinds of fish. There is no way for scientists to disprove that something could have evolved into a Sea Serpent.
Whether the Sea Serpent would have come up to the surface of the water is questionable, but it is possible that something like the Barracuda could have evolved into something like it. Maybe even eels, too, since they, again, can grow to be large sizes. Again, we have not found all the creatures of the ocean, and probably won't in the near future.
Living habitats would be hard to find if the creature was as big as some myths have it. However, the creature could be fifty, sixty, or seventy yards long and still be able to fit near the bottom of the oceans. Whether or not the creature could find enough food to sustain its large self is something else, but we do not know what it eats or how much calories it would need from that diet.
Finally, these dragons could not possibly have wings. First of all, the water would weigh them down considerably, so that they would not be able to get into direct flight from the water. And, if they are a sea dwelling creature, if they dry they would die. So, really, there would be no wings, and the creature would no need them, either.
Hydras are perhaps the most improbable of all dragons. As I have said before, the mass majority of creatures have four limbs; either four legs, two legs and two arms, or two legs and wings. Not only are Hydras depicted as having six limbs, but also more than one head! Could you imagine that? I know that I fight with myself enough, imagine if there were two!
Some people say that Hydras could be a Siamese twin of a western dragon. If you read the section on Western dragons, you would most certainly of noticed the highly unlikeliness of their existence, so it is a poor argument to give. However, if Western dragons were proven, it most certainly could be that two were combined together. Again, though, it would be more like a mule, not a real species due to lack of reproduction. Siamese twins are rare, just imagine them coming from a race that is extremely rare!
Another thing that Hydras propose is that two or more minds can live in one body. Each head would have its own brain, and in most mythology those heads each operate individually. (Which helps the valorous knight, by the way, because they argue over so much.) Now, we all know how hard it is to live with someone, just imagine if they were around you all the time and you even shared the same body! Surely, no creature could with stand that.
Finally, Hydras are too unscientific to be proven in real life. Dragons are hard enough to prove, and trying to explain how a non-proven and questionable creature can also have another mutation is simply unthinkable.
Wyverns are usually disregarded by dragon lovers. A lot of them do not think them to be "real" because they are much different. You see, instead of four legs with a powerful body, wyverns have long, snake-like bodies with two legs, two enormous wings, and a long tail. Wyverns are the most plausible of the land and air dragons due to their relationship with birds.
Now, lets take a bird. An eagle, for example: it has a small, compact body with large wings, two feet, and a jaw for carrying things. Wyverns are much like birds, especially the smaller ones that are sometimes used in stories. The larger ones are questionable, but, otherwise, they are very possible. A wyvern has four limbs, not six like other dragon types, so that mystery is solves, and they also are depicted with large wings. Perfect, a small body with bigger wings to support flight.
Also, Wyverns also have the abilities of a bird. They can grasp things, like a perch or fish, with their hind claws, or they can grasp food with their teeth, too. I suppose the jaw would be a lot like that of a Tyranosoraous Rex, large and powerful enough to grasp onto prey and hold them.
However, scientists have proven that creatures evolve and gain feathers. This is because feathers fold and can flatten out to promote faster flight. Dragons would not be able to do this with these bat wings, they would have to glide or flap their wings repeatedly to stay in flight, where birds can fly without too much effort. Now, the only suggestion for dragons not evolving feathered wings is that they are cold blooded, but, if you could imagine, it is colder in the sky when it is cloudy then on the earth. Just imagine what would happen to the dragon's temperature.
Another way to think of the wings is to say that the dragons have a relationship to bats, a warm-blooded mammal with non-feathered wings. I have attempted, on several occasions, to find the origin of the bat. However, bats seem to have a mysteriously elusive background and history. Their wings have not evolved into feathers, and there is no reason why. So, if we are looking at it from this point of view, could the bat have descendant from the same ancestor as the dragons? Could it be the ancestor of the dragons? Could it be a decent of the dragons? We might never know. If you are interested in looking into this a little deeper, read about Vertebrate Flight.