Apparently Pokemon Can Be Pretty Dark
Cubone wears the skull of its mother.
Gothitelle cries when it learns how long its trainer has left to live.
Banette is a doll that holds a grudge for being thrown away and seeks the child that disowned it.
Froslass is the spirit of a woman who froze to death on a mountain.
Absol can sense when a natural disaster will happen, and went into hiding as it was hunted for its horn, which was thought to be able to sense these disasters. Now it appears when a disaster is coming to warn people, but they think of it as a doom bringer.
Mawile is based off of Futakuchi-onna
Any others you got?
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There is aslo the Driflbim and Drifloon pokemon, which are pretty dark, id look at thier pokedex entries.