Now about my answer...
Spoiler
To me, I personally think Dragons were probably quite real at one point in time... Dead now, that's for sure. Why? Because of two things.
1: Dragons are the only mythical beast, that I know, that isn't base on any one or more specific animals (Example: Medusa is a combination of a human and snake), and wasn't made up from human lack of knowledge (Example: Vampires were made up due to humans not knowing that certain aspects of the body still grows after death and such). Dragons are just lizards... Which is actually a sub-group of Reptiles, not a specific animal. I really can't see what lack of what knowledge would make someone think of dragons and easily can tell that dragons are not base on any specific animal at all... So yeah. Any other mythical beast I can see how they were made up.
2: Dragons are common in ancient civilization all over the world and at times are recorded like they were actual animals back in the day. I highly doubt this is a coincidence. Another interesting thing, each type of dragon is slightly different though still goes with the normal 'giant flying lizard that breathes fire' deal. Just like how all humans around the world are quite similar in many ways but still different due to living in different areas of the world. (Examples of different dragons? You have Western Dragons in Europe. The classic normal ones we all think about with castles, knights, and princesses. Wyverns seems to be in old nordic cultures... I believe the dragon mentioned in the Beowulf tale was described as a Wyvern {Maybe not called a Wyvern}. China's Eastern Dragons which still have 4 legs and 2 wings but more of a snake-like body... Dragons are also mentioned in the Bible a few times and I do believe some ancient cultures both in North and South America had mention a dragon-like creature as well)...
So ye'h. Those two things are what really made me think why dragons were possible, at one point in time, alive on this world. Now, how would a creature that big fly and breathe fire? Well... I did some research into that a while ago and here is what I figured. A dragon would probably have 4 stomachs, like cows do, and actually only use 1 to digest meals. The other 3 will be filled with a specific kind of gas that can help it fly easily in the sky. Never heard of gas like this? Well... I read somewhere that there is a kind of pigeon that looks like it is too big to fly with its wings but due to having this same gas in its stomach, it can fly. However, it is illegal to feed said birds specific foods that may have oxygen compressed inside the food some-how. Why? Well, it sounds like this gas will react to oxygen within a few seconds after meeting it and basicly burst into flames inside the stomach of the bird... Which kills the bird! Now with a dragon, it'll be different due to the dragon having 4 stomachs. Any possible food will be in a digestive stomach and said effect about those pigeons will never happen for this dragon... And what happens is the dragon will release some of the gas by breathing it out of its body (Probably most likely to fly downward)... Said gas will quickly burst into flames after leaving the mouth of the dragon, making it look like the dragon is breathing fire... (It is really... Science!). The flames will probably not harm the dragon due to said flames moving away and due to have fast dragons probably are in the sky. Now another interesting thing I've read about said gas is after it is burned, it is highly acidic. So when a dragon dies, the gas inside will quickly and easily start burning (Due to oxygen can freely enter the body now. The body is shut down after all) and the dragon will burn for a bit then the acidic effects afterwards will dissolve the remains of the dead dragon. Leaving no evidence. At least this is what I read... Do I believe this? A little, I guess... I wouldn't be surprise! I mean, we already know it is possible for gases to be acidic and for gases to burst into flames when mixed with oxygen. Right?
1: Dragons are the only mythical beast, that I know, that isn't base on any one or more specific animals (Example: Medusa is a combination of a human and snake), and wasn't made up from human lack of knowledge (Example: Vampires were made up due to humans not knowing that certain aspects of the body still grows after death and such). Dragons are just lizards... Which is actually a sub-group of Reptiles, not a specific animal. I really can't see what lack of what knowledge would make someone think of dragons and easily can tell that dragons are not base on any specific animal at all... So yeah. Any other mythical beast I can see how they were made up.
2: Dragons are common in ancient civilization all over the world and at times are recorded like they were actual animals back in the day. I highly doubt this is a coincidence. Another interesting thing, each type of dragon is slightly different though still goes with the normal 'giant flying lizard that breathes fire' deal. Just like how all humans around the world are quite similar in many ways but still different due to living in different areas of the world. (Examples of different dragons? You have Western Dragons in Europe. The classic normal ones we all think about with castles, knights, and princesses. Wyverns seems to be in old nordic cultures... I believe the dragon mentioned in the Beowulf tale was described as a Wyvern {Maybe not called a Wyvern}. China's Eastern Dragons which still have 4 legs and 2 wings but more of a snake-like body... Dragons are also mentioned in the Bible a few times and I do believe some ancient cultures both in North and South America had mention a dragon-like creature as well)...
So ye'h. Those two things are what really made me think why dragons were possible, at one point in time, alive on this world. Now, how would a creature that big fly and breathe fire? Well... I did some research into that a while ago and here is what I figured. A dragon would probably have 4 stomachs, like cows do, and actually only use 1 to digest meals. The other 3 will be filled with a specific kind of gas that can help it fly easily in the sky. Never heard of gas like this? Well... I read somewhere that there is a kind of pigeon that looks like it is too big to fly with its wings but due to having this same gas in its stomach, it can fly. However, it is illegal to feed said birds specific foods that may have oxygen compressed inside the food some-how. Why? Well, it sounds like this gas will react to oxygen within a few seconds after meeting it and basicly burst into flames inside the stomach of the bird... Which kills the bird! Now with a dragon, it'll be different due to the dragon having 4 stomachs. Any possible food will be in a digestive stomach and said effect about those pigeons will never happen for this dragon... And what happens is the dragon will release some of the gas by breathing it out of its body (Probably most likely to fly downward)... Said gas will quickly burst into flames after leaving the mouth of the dragon, making it look like the dragon is breathing fire... (It is really... Science!). The flames will probably not harm the dragon due to said flames moving away and due to have fast dragons probably are in the sky. Now another interesting thing I've read about said gas is after it is burned, it is highly acidic. So when a dragon dies, the gas inside will quickly and easily start burning (Due to oxygen can freely enter the body now. The body is shut down after all) and the dragon will burn for a bit then the acidic effects afterwards will dissolve the remains of the dead dragon. Leaving no evidence. At least this is what I read... Do I believe this? A little, I guess... I wouldn't be surprise! I mean, we already know it is possible for gases to be acidic and for gases to burst into flames when mixed with oxygen. Right?