Victini

Unlike every single Legend or Mythical released prior to the Fifth Generation, there had never been one that was given the first entry in its Regional Pokedex until Victini. It was the first Mythical Pokemon to do this, as well as the first to be released in an event the date of the release of its generation's Games.

Victini is the fifth Mythical Pokemon with its base stats each sitting at a base of 100 and the first Fire / Psychic Pokemon known to man, and it's said to be the embodiment of Victory. If a trainer has obtained a Victini, it is said that the trainer will never lose.

#494 - Victini: Another Mythical Pokemon, which means it can only be obtained by a Pokemon Event. However, thought this be true, Victini was also available to be caught in-game on Liberty Island in Pokemon Black and White if you received the Liberty Pass Event.