User blog comment:FairyRaven/Fairy Tail Chapter 365: Fairy in the Jail!, Review and Commentary/@comment-5598380-20131230042329/@comment-7418318-20131230124813

We neither know what "natural" Demons are nor what the Zeref Demons are, so I don't think we can rule out anything.

For all we know the "natural" Demons might be descendants from the Zeref Demons, or actual Zeref Demons who either manged to break their bonds or were thrown out for being imperfect creations.

Or on the other hand Zeref might have enslaved natural Demons and tied them to a book, rather than outright creating them.

The only Demon where we can be (almost) sure it was created by Zeref is Lullaby (though he might have also sealed up an existing Demon into the flute), but with it's turning into an inanimate object, it's obviously a different kind than the Tartarus Demons.

And about the Satan-Soul Take-Over forms we know nothing at all.