Sinnoh Starters

In PTD4 in the first level Rowan's Lab, you can choose from 3 level 5 starter Pokemon, Turtwig, Chimchar, or Piplup.

According to a leak from cadetheslowpoke, you will have a 0.01 chance of getting these starter's shiny. And also you have 0.0125 of getting arceus as one of these starters. A 0.0001250 of getting the arceus shiny