http://www.serebii.net/index2.shtml
Yeah, it's obtainable through only this Wi-Fi event item called the "Liberty Ticket", according to Serebii.
Also, it's #000 in the Isshu Pokedex, apparently. o_O
Correct me if I am wrong but wasn't 000 a spot for Missingno? If so, then... ehh, I don' know.
So that's 6 mini-legends so far, Mew, Celebi, Jirachi, Manaphy, Shaymin, and that, which doesn't have a US name yet.