The new Arceus event in where else can legally be shiny as of Gen 6. This is the event tie-in to the upcoming Hoopa movie.
There are 19 possible random Wonder Cards for Arceus when the code is used (17 each with a random Plate, 1 with Silk Scarf, and 1 Shiny). It is set when the Wonder Card is received.
Not that it matters, since none of us can get one. Although I'm personally not too fond of the colour of Shiny Arceus, I wouldn't mind one really. Lol.