User blog comment:B214/Light Novel Volume 16 spoilers/@comment-23847729-20140128174106/@comment-1359322-20140128175406

We don't know, and there's no point checking the ones in the bible, cause Ishibumi already said it's all DxD original.

"Regarding 666, he’s the “Beast” that appears in John’s Apocalypse alongside Great Red (the Red Dragon). Also, its name and the setting is D×D’s original."