Dragon

From Wiki.ADnD.com

This is a creature found in many myths and legends. In the orient these creatures are mostly from myth, although they also appear in some folk tales. In the west these creatures are mostly found in folk tales, although a few are present in some myths. Other civilizations have had their version of dragons. The wide presence of these creatures in the spoken histories has had many seek some verification of them having once existed.

Most dragons are considered reptiles, although certain facts about them clearly state that they are not. The lower jaws of dragons are almost always thick with dragons actually having chins. Dragons have an extra set of appendages as their wings (it is Wyverns that are generally noted as having their front arms as wings, which actually makes Wyverns more believeable as possibly real creatures). Dragons, especially the older ones, are generally considered intelligent and capable of speech.

1ed Dungeons and Dragons
There were numerous dragons and dragon types presented. Certain rules however did seem to be maintained:
Colored dragons were evil.
Metallic dragons were good.
Earth dragons were neutral.
That was it. It was left up to the DM to determine which dragons or dragons types he allowed into his campaign or developed on his own.

3ed Dungeons and Dragons
The basic rules of 1ed dragons have been maintained. Dragons are however noted to be immune to sleep and paralysis. They are also assumed to have low-light vision and darkvision with a range of 60 feet. Most dragons are also assumed to recognize the presence of even those invisible.