I'm looking at Fairy Tail Zero as a prequel. It has a main cast different from the regular TV series. It might be a separate arc, but it is unique enough to encompass a whole season, like most separate series. Also, it has been advertised as Fairy Tail Zero, even in the title sequence. If it was just another continuation arc, they would have titled it normally as Fairy Tail, with each episode titled "Zero Arc" like it was done with Tartaros Arc.
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