Near the end of the Volcano course, you'll encounter a large, glowing egg. This is the Moltres egg, and you need to hatch it to get a photo of the legendary bird.
To hatch the egg, throw Pester Balls at it repeatedly. It will take several hits, so be persistent. Once it cracks open, Moltres will emerge, flying majestically. Take as many photos as you can while it's in the air for high scores.