Dialga, Palkia, Manaphy, and the Trio are not the only Legendary Pokemon that inhabit Sinnoh. You'll be able to encounter at least four more Legendaries post-game, as well as another rare one-time-catch.

CRESSELIA

Fly back to Canalave and visit the house next to where you can get on the boat to Iron Island . Inside, you’ll find a mother with her sick son, who has been sleeping and is unable to awake. There’s a mythical pokemon who can provide the cure for this strange affliction, but it lives on an island far out at sea.

We can’t let a little thing like an ocean stop us from our heroics! Head back outside and talk to the boy’s father (the sailor). He’ll take you to Full Moon Island.

There isn’t much to see on Full Moon Island, so you can just head straight for the forest clearing. Inside, you’ll catch a glimpse of the beautiful Cresselia, the Legendary Crescent Moon pokemon. It will vanish, but she leaves behind a silver feather for you, which is the item you need to awaken the sick child. Press A to pick the feather up, then head back to your boat and show the feather to the boy’s father.

You'll head back to Canalave. When you come ashore, head back to the family's house and use the feather on the sleeping boy. He'll wake up, and everybody will be happy again. Awwww, don't you feel warm and fuzzy now?

But where did Cresselia go? She's actually roaming Sinnoh, testing your ability to find and capture her. Much like Mesprit, Cresselia can be tracked by using the Marking Map Poketch application. (If you still have Mesprit floating around uncaught, be warned that the icons representing Cresselia and Mesprit look exactly the same, so be prepared to encounter either one.) She also is very quick to flee, though she doesn't heal herself between encounters. She's considerably stronger than the Legendary Trio, though her Psychic type makes her particularly vulnerable to the usual Bug, Ghost, and Dark attacks. Once its HP is low, focus your attention on trying to capture it. Ultra Balls work best, through Quick Balls (as the encounter begins) and Dusk Balls (at night) also have a better chance of snaring her. Using skills that cause paralysis and sleep conditions will also help.

Or, if you want to save yourself a great deal of frustration, time, and headaches, use a Master Ball when you first meet up with her again. This is the sort of pokemon those things were designed for.