Fairy Tail Zero getting an anime
I am not sure if this part of the main Fairy Tail anime or a seperate series. My guess after the Tartarus, this is getting its time slot.
So either it just episode of the 2014 episodes, or officially the 2014 anime will end in December, and the Fairy Tail Zero anime will take it place, and after it runs it course, we will either adapt another spinoff like Ice Trail, or the main Fairly Tail anime which will now officially be the third Fairy Tail will relaunch.
If they do that, I am sure Funimation will still all consider it one big series. They consider the original 175 episode anime and the current 2014 anime as one, so they will likely consider the same thing as well.
I highly doubt this means the end of the main anime. My best guess from what I can gather and have read elsewhere, is that Fairy Tail Zero, being a fairly short series of only 13 chapters, will be used as a sort of filler arc between the Tartaros and Arcs to give the manga a chance to get ahead some more. I have not read or seen the Tartaros arc yet, as I wait for the dubs, but, from what I have read, there is a point at the end of the arc where the Fairy Tail Zero material would fit quite nicely as a sort of flashback. After they finish the FTZ stuff, I would imagine the main storyline will continue on as normal, like it has after every filler arc. The only difference between FTZ and say, Key to the Starry Skies is that FTZ wouldn't be anime original material.
Either that, or they simply run the two series simultaneously. It could also just be a few OVAs packed in with merch in Japan or something else. In that case we likely wouldn't even get it in this part of the world. We have no real idea at this point because all that has been said is that there will be an anime adaptation of FTZ and not much else.