However, these 4 Pokemon are in BW2, and you'll need some unique Trading: http://www.serebii.net/black2white2/unovalink.shtml
In all the games that offer it, Regigias won't "appear" unless you have Regirock, Regice & Registeel in your party.
The issue is that in Diamond and Pearl, that Regirock, Regice & Registeel needs to be transferred from a 3rd Gen game. Platinum allows you to catch all of as long as you've got the event Regigias, which you can trade for. http://www.serebii.net/platinum/regigigas.shtml
Well, Jirachi and Deoxys where given out for events in the 4th Gen, but both are over now.
I'm not sure if Phione is counted in as an Event Pokémon, but there are three ways to obtain it. Trading, Pokémon Ranger games with any 4th Gen game, and Breeding that Manaphy, and My Pokémon Ranch with Diamond or Pearl.
Anyway, I'm trying to work out which Pokémon can change their Formes in a battle. Ditto, Mew and Smergle via Transform and Zorua & Zorark via Illusion. Castform due to weather. Cherrim is due to Sunny day effects. Gratina (item) and Shaymin (sky to Land via status) and Zen Mode Darmanitan, Arceus (items) and Meloetta. Am I missing anyone?
Deoyxs is not counted since you can not change it's form on in battle.