I think Ghost, Fairy, and Dragon all have the same design direction. In that they're all based on the artstyle rather than unifying themes.
Ghost-type Pokemon look mischievous (there's an awful lot of squinting and smirking) and amorphous. They usually lack feet.
Dragon-type Pokemon look like powerful kajiu. They're vaguely reptilian.
Fairy-type Pokemon look cute and amorphous. They're vaguely humanoid.
I don't really think any of them has hewed very close to the mythological origins of their types, basically using that as a starting point.