The Masquerade Pokémon
It is capable of disguising itself as anything it likes. Although it appears to be a bird Pokémon, this is just a disguise. No one has seen its true form.
Although it is very beautiful, it is also sinister. It puts its foes into an eternal sleep and steals their souls to add to its tail.
quail/peacock/venetian mask fakemon that are also based on a certain character from a certain sequel to a certain game about pigeons. mind the spoilers. their ability is Illusion, like Zoroark.
(these guys will be in Uranium, although not in the latest beta; they are the signature Pokémon of the 6th gym leader.)