Cresselia

Cresselia

Assuming you've gotten the Lunar Wing from the Strange House outside of Reversal Mountain, after beating the game, you can visit the Marvelous Bridge just to the east of Nimbasa City. You can actually visit the bridge without the wing, but if you have the Lunar Wing in your inventory, you'll encounter Cresselia on the bridge, ready for you to catch it!

Marvelous Bridge
Cresselia Cresselia  Lv. 68