I have a Latias in Pokemon White 2, and when I send it out, a yellow sparkle appears above it's head. It's in a masterball, but this has only started happening recently. It doesn't have Pokerus, nor is it shiny, so what's happening?
Did you recently particpate in the Pokestar Studios with it? If so, you might have achieved a Strange Ending. This gives the Pokemon who participated in the film a special animation and shine.