Question

Why is Zacian a fairy-type ?

Answer 1

For one, Zacian is the sister of Zamazenta, and the Fairy type has established itself as a feminine type in Pokemon.

In addition, Zacian's feminine nature may be a reference to the Lady of the Lake

The Lady of the Lake is a name or a title used by several fairy-like enchantresses

Which further supports why it's a Fairy type.

Source
Source 2

Hope I helped!

Comments

Zacian is a she ???

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Yep. It's been confirmed that, while they do not have genders, Zacian is the sister to Zamazenta, and Zamazenta is the brother to Zacian.

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OK thanx =)

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You're welcome!

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Oof.  Sniped.

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Sorry, Jhnfui.

Answer 2

It says in the description that zacian's graceful movements captivate opponents, like a fairy entrancing/hypnotising someone by dancing as happens in some traditional stories. Other very graceful Pokemon are also fairy type, such as Gardevoir and hatterene.

Comments

thanx u too =)

Answer 3

In addition, Zacian's feminine nature may be a reference to the Lady of the Lake, the character responsible for giving King Arthur his Sword.

Now, On Wikipedia page for the Lady of the Lake, it says that it is a name used by several Fairy like enchantresses. So, this is probably why.
Source: https://m.bulbapedia.bulbagarden.net/wiki/Zacian_(Pokémon)

Comments

thanx u too =)

Answer 4

Zacian is Zamazenta’s older sister, and Zamazenta is Zacian’s younger brother. Fairy is a more feminine type, plus since she is based on Excalibur, her type could be because of Lady of the Lake.
The Lady of the Lake gave King Arthur the Excalibur. Plus, she was asleep in stone in the Slumbering Weald, which has fog, and foggy weather/mist is sort of related to the Fairy type.

And, mainly, because Game Freak said so!