User blog comment:Ariel Saeba/Mythology and Character Origins in DxD/@comment-3151468-20160402061151/@comment-27232228-20160402111458

As an interesting note, the popular depiction of Tiamat as a dragon (especially the multi-headed version) actually stem from games; the most popular being Dungeons and Dragons and Final Fantasy Series, inspired from sources that mention her specifically as a dragon. DxD also cites this source, but added the twist of her being capable of shapeshift to a beautiful woman with blue hair based on the first myth version where she is said to have a peaceful creation myths, representing feminine beauty (the "sacred marriage" and "primordial waters" aspect). Whether Ishibumi will play her deitical/God factor to kick in depends on the final arc (especially as she is considered the strongest Dragon King in the story, which is very much justified from loooking at her myths).

Oh, and most of the deities/gods you mentioned are popularly depicted with a snake form, but whether they are considered dragons or not depends on the media/fiction they appear in.