In geneations before 6
You have a 1/8192 chance to encounter a shiny Pokemon, this includes legendaries.
Edit: Zekrom, Reshiram and Kyurem are also shiny locked in generation 5 games(thanks to Dan for pointing it out).
While the chance to find shiny Pokemon is halved(1/4096), all the legendary Pokemon in X/Y are shiny locked(meaning you can't get them in their shiny variants).
Sources: this and knowledge
Hope I helped.