Unobtainable Shiny Pokémon
In most generations there are a few Pokémon that cannot be obtained Shiny without cheating or glitches, due to either being distribution-exclusive without an in-game location or, in Generation V, being programmed so that they cannot be Shiny in the wild.
Unown forms other than forms I and V
No, they cannot be shiny. The theory is that Nintendo/ Gamefreak wanted you to actually play the game, rather than soft-resetting a million times to try and get a shiny legendary.
Bulbapedia on shiny Pokemon