Pokémon evolving with their trainers. I had my own idea similar to this for Halloween someday, with a cosplay-like thing based off one of my favorite Pokémon. I had Dratini, Riolu, Dragonite, Lugia or Torchic in mind.
Pokémon - What Comes Next? Technically, the last two are wrong.one is supposedly death (draining life, Yveltal) and the other is life (Xerneas. and the Tao Trio (Reshiram, Kyurem, Zekrom) the duality was Ideals and Truth
Pokedex entry: "Blissey senses sadness with its fluffy coat of fur. If it does so, this Pokémon will rush over to a sad person, no matter how far away, to share a Lucky Egg that brings a smile to any face.