Page 1 – Arceus | Articuno | Azelf | Celebi | Cresselia
Page 2 – Darkrai | Deoxys | Dialga | Entei | Giratina
Page 3 – Groudon | Heatran | Ho-Oh | Jirachi | Kyogre
Page 4 – Latias | Latios | Lugia | Manaphy | Mesprit
Page 5 – Mew | Mewtwo | Moltres | Palkia | Phione
Page 6 – Raikou | Rayquaza | Regice | Regigigas | Regirock
Page 7 – Registeel | Shaymin | Suicune | Uxie | Zapdos 

This Iron Pokemon is the Steel-type third of the golem trio introduced in Ruby / Sapphire.

Obtainable via:

Pokemon Ruby / Sapphire / Emerald – From Lilycove City, head west and go up two flights of stairs and you’ll reach the Ancient Tomb. The message inside here is rather cryptic, but head to the center of the room and use Fly for Ruby and Sapphire and Flash for Emerald. The door to Registeel (level 40) will open.

Pokemon Platinum (North America and Japan only) – Event only! To unlock the golem trio (Regice, Regirock and Registeel) in Platinum, you’ll need to have obtained the special North American event Toys R Us event Regigigas (or if you're playing on a Japanese version, the movie theater event one).

You’ll have to transfer Regigigas from your Diamond or Pearl cartridge to your Platinum cartridge. With the special Regigigas in your party, head to Iron Island (you get there by taking the boat from Canalave City). You’ll find the level 30 Registeel on the 3rd floor. Again, step on all the dots to trigger the battle.

Trivia: Registeel is the only pure Steel-type legendary, and one of only two pure Steel types among all Pokemon, the other being Mawile.

This adorable Grass (or Grass/Flying) type Gratitude Pokemon is one of the Pokemon that has alternate forms introduced in Pokemon Platinum.

Obtainable via:

Pokemon Diamond / Pearl / Platinum – By event only. There’s currently no way to obtain Shaymin through normal gameplay. In Pokemon Platinum, Shaymin can morph into its Sky Forme during the day whilst holding a Gracidea, which can be obtained in Floaroma Town from the blonde lady outside if you have an event Shaymin in your party.

Trivia: In Shaymin’s Land Forme, it’s the smallest legendary Pokemon (not to mention the only pure Grass-type legendary). So cute!

Suicune is the Water type third of the legendary beast trio introduced in Pokemon Gold / Silver.

Obtainable via:

Pokemon Colosseum – Suicune is available to capture during the boss battle with Cipher Admin Venus at The Under.

Pokemon FireRed / LeafGreen – If you chose Charmander as your starter Pokemon, Suicune will appear roaming around Kanto after you beat the Elite 4.

Suicune is the only legendary beast who doesn’t know the move Roar, so if you picked Charmander as your starter, you’ve made your life much easier. When you encounter him, use the usual tactics for roaming Pokemon (as in, the tactics that won’t work for the other two legendary beasts). Prevent him from fleeing using a Wobbuffet or a Pokemon that knows Mean Look, and he should be fairly easy to catch.

Suicune is still an excellent candidate for using your Master Ball, since all roaming Pokemon are generally a pain to catch.

Trivia: Suicune is the only non-Ice type Pokemon who learns Blizzard through leveling up, at level 85 in Platinum.

Uxie is part of the Psychic legendary trio that debuted in Pokemon Diamond / Pearl. Each one guards one of Sinnoh’s three lakes.

Obtainable via:

Pokemon Diamond / Pearl / Platinum – Once you’ve beaten Team Galactic at Mt. Coronet, the lake trio of legendaries (Azelf, Mesprit and Uxie) will become available. Find Uxie at Lake Acuity.

Trivia: Uxie is the mascot of PokePedia, which is a French language online Pokemon encyclopedia.

Zapdos

One of the three legendary birds from Generation I, Zapdos is the Electric/Flying type third of the trio.

Obtainable via:

Pokemon Platinum – Once you get the National Dex, the three legendary birds (Articuno, Moltres and Zapdos) will be roaming Sinnoh in the same way that Cresselia and Mesprit jumped around from place to place. Use your Marking Map Poketch app to see their current locations and track them down. Once the encounter begins, use a Pokemon with the Mean Look move to prevent each bird from fleeing (or better yet, use a Wobbuffet – its Shadow Tag ability will automatically prevent the bird from fleeing, even if you switch it out) and you should have all three in no time.

Pokemon FireRed / LeafGreen – Zapdos can be found at the Power Plant. To get there, fly to Cerulean City and head to the Pokemon Center near Rock Tunnel. Go north and use Surf when you get the river. Follow the river until you get to the Power Plant. Make your way through the building and you’ll find Zapdos inside.

Pokemon XD – Zapdos can be caught during the boss battle against Grand Master Greevil on Citadark Island. This is a pretty tough boss battle and it’s fairly difficult to catch the legendary birds this way. Since the bird trio is now available on Platinum, there’s really no reason to go back to XD, unless, of course, you really want to.

Trivia: Zapdos is the ONLY dual-type Electric/Flying Pokemon.