Cynthia is a Chansey.

I know this sounds completely insane, but think about it. Remember that one myth in Canalave's library that talks about a Pokémon that turned into a human? I am 100 percent certain that said Pokémon is, in fact, Cynthia. She mysteriously demonstrates the ability to heal your party to full just by holding them (with the Pokémon Center noise, might I add,) after the Distortion World, not to mention the magic egg full of "happiness" and the Secret Potion for the Psyducks. Not only that, but she frequently pops out of tall grass to speak with you, has an egg charm on her jacket, and has those... head... doodlebobbers- all rather reminiscent of the common Chansey, wouldn't you say? I rest my case.