You need the Eon Ticket, a Nintendo Event item that was given out years ago. Go to a boat in either Slateport or Lilycove. When you arrive at the island, you will find the Latios in the forest part. You can catch it at level 50, leaving an Egg. If you defeat it, you can take the Egg to raise the Latios from level 5. Please tell me if I'm wrong about the Second part since I haven't tried it.