The Galar Heroes transformed.
when I came up with this one I had assumed it was already kind of implied in the games if so if you agreee consider this more of a deep read between the lines.
I firmly believe that the two human heroes in the tale of the original darkest day didn't just summon Zacian and Zamazenta they became them. They somehow imbued their sword and shield with Eternatus' power to use against it but had to become Pokémon to use that power.
Here are our clues:
#1 If (z&z) already existed at the point of the 1st darkest day there would be no need for human intervention in the first place they could have just stopped it and the legends would only refer to them.
#2 At the point of the game where you fight Eternatus you're Pokémons' moves don't work just like when you face (z&z) and only start having some effect when (z&z) show up and really their moves are the only thing that can make significant impact.
#3 the two heroes kind of mysteriously dissappear "sail off" and all of their statues and history gets shrouded and buried some depictions shoe only humans some with the beasts and some just (z&z) alone. Yes we meet their descendants later but that doesn't mean their exactly direct descendants my sister's kids could count as descendants.
#4 any other Pokémon that wields a weapon or tool like that carries it in their hand if (z&z) where naturally occurring they wouldn't have likely evolved to carry a sword or shield in their teeth.
In conclusion, I believe two humans attempted to stop the darkest day when they where unsuccessful they imbued their weapons with wishing stars in another attempt to harnes that same power, but humans can't wield Pokémon powers so by either devine intervention or their own self sacrifice they transformed into the legendary Pokémon and won. Unfortunately they where unable to transform back to humans. Unable to return home as beasts they retired to the Slumbering Weild waiting for the day they'd be needed again. After hundreds of years legend and myth get intertwined families stop talking about the two relatives that became dogs and there you go.