Meloetta/Meroetta (メロエッタ) is one of the new event legendary in Pokemon Black and White and will likely have a starring role in a new Pokemon movie.
Similarly to some legendary Pokemon in the past, Meloetta also has a alternate form in the games, but unlike most of them, Meloetta’s alternate form also has a type change.
Its normal form in the games is the Aria form, which is Normal/Psychic, while its Piroulette form, which can only be obtained when it is taught the move, Relic Song, is Normal/Fighting. It’s been speculated that the special move, Relic Song, can only be taught to Meloetta through a special event which leads me to believe that a special event will be made available in a future Pokemon movie in Japan to activate Meloetta’s alternate form.
Pokedex entry: It sings with a unique vocalization, and freely manipulates the emotions of creatures who hear its melody.