If you buy one of the games between March 6 (launch day) and April 10, you'll be able to use a the Nintendo WiFi to catch the rare critter. Nicknamed the Victory Pokemon, Victini is a Fire/Psychic type. It has a Victory Star ability that boosts the accuracy of all Pokemon in a given battle, making it especially useful for the new Triple Battles.
As a rare type, Victini isn't available through normal play. You'll have to grab him at launch, or strong-arm a trade with someone else who did. And who would willingly give up a fox-chicken-bat that shoots fire and plays with your mind?