User blog comment:BlackWizardZeref/Double Review: FT Chapter 416 & FT Zero Chapter 7/@comment-24039250-20150120045531

Theory Time: Igneel and END fight a grueling battle 400 years ago which resulted in END losing. Before Igneel can deliver the final blow, END being out of power reverts into a different form, a small human baby. Igneel seeing this innocent child decides on a whim to NOT kill END. Zeref appears and tells Igneel that he must take care of the child and raise him to become strong enough to kill him (Zeref). Igneel agrees to this and Zeref decides that in order for the child to be safe he should be sent to a different time where there is no war, but instead peace where at the same time Zeref and Igneel will still be alive, so Zeref sends Natsu via Eclipse/Time Magic 400 years into the future where Igneel will one day find Natsu and raise him as his son. Before sending him into the future Zeref gives Natsu the name he will be known as by his nakama: Natsu Dragneel (Dragneel as homage to Natsu's future foster father, Igneel (DRAGon-inNEEL)). As a demon he will carry the name Etherious, so his full name will be Etherious Natsu Dragneel AKA END!!!

Let me know what you think of this theory. Maybe I'll make an extended version blog of this.