User blog comment:IamJakuhoRaikoben/Chapter 449: Mavis & Zeref, Review./@comment-1359322-20150824060457/@comment-9595671-20150824065239

Considering how close the anime's getting to the manga, I wouldn't be surprised if they decided to animate it. Seeing as at this point the story, Fairy Tail Zero's story seems really important.

Then again, maybe they'll animate Fairy Tail Zero from the point where she begins talking about the past too. Either way I think we'll see Zero get animated as Mashima himself worked on it.

Its part of the main story, but just not in the main manga like the Key of the Starry Sky arc. The main question we have right now is when it's simply gonna be added into the anime.