I say check Gamefaqs.com. They should be able to help you out. Might learn something 2.

You can get the pokemon Cressilia, only when you beat the entire game.I encountered it at one of the islands, but it escape. The bad part is.....you have to go after it, in the wild.....just like any other legendary Pokemon, unless you have a Master Ball.

You get cresselia by going to Sailor Edriches' (don't know spelling) house (after beating the Elite 4 and obtaining the national dex) and talking to the people inside. You will learn that the sailor's son is sick, and he needs you to get a certain item from full moon island to cure his son. He then takes you to full moon island, where you encounter Cresselia. However, you do not get to battle Cresselia; it flies away when you walk up and press A on it. It will leave behind the item you need, and you then go and cure the sailor's son. Now, as for Cresselia, it will be running randomly around the Sinnoh mainland. You will be able to track it with the marking map application on the Poketch. Every time you switch routes or move from a town to a route Cresselia will move to one of the routes next to its current location. When you use fly, it will randomly teleport to a new spot on the map. YOu will be able to encounter it if it is in the same route as you and you run around in the grass. However, it will flee from you unless you catch it on the first turn, put it to sleep, or use mean look/block or some other trapping move. Also, if you send out a pokemon with Arena Trap or Shadow Tag Cresellia will not be able to flee. However, it may know teleport, which will allow it to escape from all of the above execept if you are faster than it and put it to sleep immediately. However, this will only give you 2-5 turns to get its HP down and catch it, which is exteremly difficult. Therefore, the best option is just to use a masterball when you encounter it. Hope this helps you get it. Good luck and good catching.