User blog comment:BlackWizardZeref/Fairy Tail: Chapter 436 Memoirs Review with a brief mentioning of Chapter 435 Victory Shout!/@comment-26416024-20150519220151

I think that Mashima came up with Natsu's last name using the words dragon and igneel but since he never declared that officially, he decided to retcon that and coincidentally make it his real human last name once he came up with E.N.D.