Mew

Mew is a Mythical Pokémon, reputed to be the genetic ancestor to all living Pokémon.

History
Mew is perhaps even more rarely discussed in myth and legend than Kanto's Legendary Birds. Very few people know of its existence, and even fewer have seen it. At some point, genetic material from it was found and used to create Mewtwo.

That doesn't stop Cira, who has made it her mission to find Mew. Only she knows why.

Mew has appeared at least once explicitly in canon: [THEORY]
 * Mew played with Honey Babe, Serenade, and Ripley, and its glitter turned them into Shiny Pokémon.

There have been a few other occasions where Mew likely interacted with Rose and Cobalt's journey:
 * Mew likely left the Prismatic Powder on the Red Monument in Pallet Town, of which Rose collected a sample. (001)
 * Mew likely turned into the Clefairy that Rose and Cira used to gain access to Operation Fight Club.
 * Mew likely was the Clefairy that saved Baritone, and protected the Mysterious Egg, from the Darkness Beasts in Mount Moon.

Description
Mew is a small, pink, creature that can be described as a cross between a cat and an embryo. It has large, adorable, blue eyes and sometimes sheds a cloud of glittery particles. As a Mythical Pokémon, it has incredible powers that it uses for a variety of unknown reasons.

Trivia

 * Mew is Shannon's favorite Pokémon by a long shot.