Zygarde will not have the Ability Power construct in GEN-6 games(XY) as the ability was introduced only in GEN7.
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Look here in the ability section. There are 2 abilities for zygarde. 99.99% chances are that you got aura break as your ability. This is nothing to worry about as zygarde can have either abilities. If you have caught it and want power construct, you have to get it again. As you haven't specified where you got it, I can only tell this much.
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