User blog comment:Besteverunited/Do you guys think that Natsu is the only demon Zeref created or do you think the other dragon slayers were created by him to?/@comment-26460727-20150604090335/@comment-5.64.180.102-20150606005358

There was also an episode in the Edolas Arc where Natsu,Gajeel and Wendy pretend to be demons so that could be foreshadowing aswell.