Lakeshore Village
If you want to find and catch the "Arceus" Pokemon, head over to Lakeshore Village, which is Map 6. Explore the area until you find a secret cave. Do a little bit more exploring and you should find the "Arceus" Pokemon. In order to catch him, you must first battle him with one of your own Pokemon, so make sure that you bring a battle-worthy opponent to fight against "Arceus." Weaken him enough and catch him with a Great Ball.
Research Lab
Got to Map 6 and find the "Research Lab." There you will find and have the opportunity to catch the "Zapdos" Pokemon. Zapdos is a rare flying Pokemon that was featured in the original "Pokemon" television series, and will be a great addition to your team.
Ember Cave
There are three different Legendary Pokemon that you can catch at Ember Cave. They are: "Ho-oh," "Groudon" and "Heatran." Walk around the cave and you will encounter many different types of Pokemon depending on where you travel. This may take some time, but walk around until you come face to face with all three of these Legendaries.
Snowflake Village
At Snowflake Village you will find the Pokemon "Articuno" and "Plasma Articuno." These are rare bird-like Pokemon that have tremendous attack abilities. In order to find the "Plasma" version of "Articuno," you must walk around the Snowflake Village for five hours. This may seem like a long time, but it could be a powerful addition to your Pokemon collection.