User blog comment:B214/Volume 22 spoilers/@comment-73.160.58.215-20160723140310/@comment-24.113.252.213-20160724012154

The Extra Demons are (as far as we know) the names of demons mentioned or listed in demonology, folklore, or other sources.

For example:


 * Mephisto Pheles is actually a demon from German folklore (from the Faust legend). The original name of the demon is Mephistopheles (his name in DxD, but all in one word).


 * Abaddon is the name of a male Angel in the Book of Revealation. He is the King of the demon locusts sent to torment mankind. He is also the Angel of the Abyss.


 * Lucifuge, according to the Grand Grimiore, is the demon in charge of Hell's government by order of Lucifer (so the Clan's role in DxD is spot on). The original name of the demon is Lucifuge Rofocale.

I don't know the origins of the other known Extra Demons, but we can assume they are similar to the ones I listed above.