Maybe dragons, in some form, did exist at one time. However unless we find some type or fossil or remains we'll never know for sure. The argument that all those different ancient cultures had myths about dragons actually makes it less likely that they ever existed. If they were common enough for vastly different cultures in different parts of the world to have seen them then we probably would have found some sign of their existence by now. Still, like I said, there's currently no way to know for sure and lack of proof doesn't mean they're were never any dragons.