The mythical Pokemon that can be caught today in a main series game without glitches/hacks as shiny are
Celebi (Crystal Virtual Console)
And technically Jirachi and Manaphy, if you have the Colosseum Bonus Disk and Pokemon Ranger, respectively.
Therefore, the Mythical Pokemon that can be only obtained as shiny in Pokemon Go are
I don't believe the other Gen 5 shiny mythical are available in Pokemon Go but correct me if I am wrong