Prussia is a region on the Baltic sea. Russia still has a holding there named "East Prussia," if I recall correctly, though most of what is truly Prussia is now divided between Poland and Lithuania. I'm sure one of the areas actually has a flag, probably even the correct Black Eagel on white, but individual regions and states/provinces don't merit flags. However, by technicality you'd be able to argue for the flag to be included if the East Prussia controlled by the Russian Federation, as a Federal Republic, though I don't believe it is.
"It is not bigotry to be certain we are right; but it is bigotry to be unable to imagine how we might possibly have gone wrong"
- G. K. Chesterton