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So as we all know, our beloved Mimikyu's true appearance will bring death upon all who gaze on it. But why? Possibly because of terror, maybe an unknown disease, but it's not exactly sure.
So what happens here? Might be because Mimikyu has some kind of dark unwanted power, but then again, that sounds like the idea of a Fairy Dark type. So why is it Fairy Ghost? It likely isn't because it's dead, as we've seen a diseased shiny Mimikyu in the anime owned by Acerola, named Mimikins. So a ghost of a ghost doesn't make sense.
My current idea is that there are ghosts on Mimikyu's body that when people look at them, it angers them, pulling their soul on a walk to their afterlife. Why do the ghosts choose Mimikyu specifically? That I do not know. This is a theory at best, and a theory it will stay.
But still, why can Mimikyu look at itself? This has been seen with Jessie's Mimikyu looking at its own reflection on the water on its last fight against Pikachu. So how does it do that?
This is the reason Mimikyu has its fairy typing. To avoid the wrath of the ghosts that inhabit the creature, Mimikyu has adapted to use surreal powers to repel the force of ghosts.
But I have a bit more tea to spill, or coffee, because it's not canon, but I'll continue. What if the ghosts took on a physical form? As we've seen, Jessie's Mimikyu had a somewhat sad reaction to its true appearence in its reflection from the same fight. Perhaps the ghosts had stopped Mimikyu from seeing its own self, and replaced what it saw with an everlasting ugliness that Mimikyu always believe is itself, and the reason it is hated?
Once again, this is a theory. But if it gives a reason to make Mimikyu Fairy Ghost instead of Fairy Dark, then I'm all for it. There are loose parts, but I'll leave that to more theorizing.
Summary: Mimikyu seems to be Fairy Dark, but it's Fairy Ghost instead. I ruled out that it was a ghost, because there's a ghost Mimikyu in the anime, so a ghost of a ghost doesn't make enough sense to me. So I came up with: There are ghosts on Mimikyu's body that pull people who look at them into the afterlife. They also have a physical form, stopping Mimikyu from looking at its true self in the first place. Mimikyu uses fairy powers to avoid the power of said ghosts. However, Mimikyu still believes its appearance (which only it can see) is its true form because of the ghosts taking a physical form, causing it to believe that its true form is ugly (The ghost make an ugly appearance for some reason, and Mimikyu thinks it's part of why it is hated)
by According_Ad8711