In Pokémon FireRed, LeafGreen & Emerald, lies a cave named the Altering Cave.
At first glance, all it has are wild Zubat, which makes me people ask "why the heck does this cave even exist to begin with".
As it turns out, there is more to this cave than it meets the eye.
This cave was programmed to make use of special Mystery Gifts, that would allow the wild Pokémon species that appears to be altered.
Sadly, this game feature never had an event to make use of it.