How does Zygard mega evolve?

Question

Zygard could never mega evolve before, and now it just can all of a sudden? How and why?

Comments

I imagine that there might be an in-game explanation for this, but I'd like to point out that old Pokemon gaining different forms isn't anything new. It happened when mega evolution was first introduced when ORAS came out, and Zygarde's 10% and complete forms didn't exist until Sun and Moon, and likewise, some Pokemon have received additional evolutions as time has gone on. Just pointing out that it's silly to point at Zygarde suddenly being able to mega evolve and question why it can do that when this has been happening for 26 years when Onix was suddenly able to evolve into Steelix.

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Hellfire Taco could never spit facts before, and now it just can all of a sudden? How and why?

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Just like Hellfire Taco said the same question also applies to every other mega evolution and cross-gen (pre)evolutions. This has been happening since gen 2

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you're not asking the actual way to get the mega, right?

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I'm pretty sure from the shine bright like a gemstone quest, they confirmed that new mega stones were created from the light of the ultimate weapon being fired in the events of XY.

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Friendship evolutions are a scam invented by Big Haircut in Gold and Silver to sell more haircuts.

Answer 1

AZ himself supports this notion, further claiming that the recent firing of the ultimate weapon by Lysandre is the reason why new Mega Stones have been found as of the events of Pokémon Legends: Z-A.

Essentially, the theory in-game is that mega evolution came from the energy of Xerneas and Yveltal's energy when the ultimate weapon was fired 3000 years back by AZ. Since it was fired by Lysandre again during the events of the XY games, new mega stones formed, most likely including Zygarde's mega stone.

I would note that the pebble becoming the mega stone is similar to the meteorite in ORAS.

Source: Bulbapedia

Answer 2

When game freak released legends Z-A it was obvious a lot of Pokémon would get new mega forms. And one of those Pokémon is Zygarde, the main legendary for the game.
In x and y, Zygarde only had one form and got more forms (10% and 100%) in sun and moon.
A lot of people predicted mega zygarde, and made some cool designs for it and were not very surprised when it was added. Or at least I wasn’t. New forms had been happening since gen 2, e.g Onix could never evolve into steelix. The rest of what you need to know has already been said by hellfire taco in the comments

Comments

I didn't use my comment as an answer because I figured there was a plot reason Zygarde can mega evolve