Defining dragons is a very complex subject, and cannot be done with a few sentences. Even a more elaborate definition, such as this one, has problems classifying some creatures. Ancient sources of dragon information can be hard to translate, and some information may only be found in one source. This means that sources cannot always be crossed checked. Even with good sources, there are some creatures who are borderline cases, such as nagas. Nagas are from India. They are serpents, with human heads. Many have supernatural powers. Nagas would not be the first dragons to look chimeric. Having human parts is not intrinsically different from a dragon having bird wings or fish fins. Dragons already share other human traits (such as intelligence), so why could a dragon not share physical traits too? It is a matter of opinion whether there is a difference in looking like a human, to looking like a bird. Issues like this also contribute to blurring the borderlines of what is a dragon, and what is not a dragon.