The Fairy Tail Zero had finally announced that the anime adaption will be aired next year, winter season, so probably, by January 2016. The Fairy Tail Zero is a prequel to manga series, Fairy Tail. It talks about the history of first guild master Mavis on how she integrate and manage the guild by herself. Currently, it has 13 chapters (released in Novermber 17, 2015) and its still on going.
According to wikia, “On the year X679, Mavis Vermilion is left to fend for herself after a war takes the lives of those who reside on the island.Seven years later, she encounters those who will, in the near future, title themselves as the founding members of Fairy Tail“.