This Iron Pokemon is the Steel-type third of the golem trio introduced in Ruby / Sapphire.
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.
Pokemon Black / White 2 – Specific to Black 2, you'll need a key from Regirock that will allow you to alter the ruins in Clay Tunnel to find Registeel.
Trivia: Registeel is the only pure Steel-type legendary, and one of only two pure Steel types among all Pokemon, the other being Mawile.
If you chose Pokemon Black as your version, chances are Reshiram has at least something to do with your decision. This fluffy dragon looks less intimidating than its White cover-star counterpart, Zekrom, but its legendary status assures its stats are out of control.
Pokemon Black only - If you're playing Pokemon Black, you must capture Reshiram as part of the story in N's Castle after you defeat the Elite 4.
Pokemon Black / White 2 – Specific to White 2, you will receive a Light Stone from N in his castle after defeating the Pokemon League. Take this stone to the top of Dragonspiral Tower to capture it.
Trivia: Not only is Reshiram's Dragon/Fire type combo unique to it alone, but it also makes it the only Fire-type Pokemon not weak to water.
This adorable Grass (or Grass/Flying) type Gratitude Pokemon is one of the Pokemon that has alternate forms introduced in Pokemon Platinum.
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.
Pokemon HeartGold / SoulSilver – Suicune is loosely tied to the games' plots and you'll run into him a few times but will be unable to battle him as he will run away as soon as you interact with him. You'll have to persue him around various locations before he will battle you.
Suicine is at the following locations in this order: Brass Tower (with Entei and Raikou), north of Cianwood City, entrance of Mt. Mortar on Route 42, Vermillion City in Kanto, Route 14, and finally outside of Bill's House on Route 25 after Misty has returned. At this encounter you'll finally have a chance to capture him. Suicine will be level 40 for this encounter.
If you defeat him instead of capturing him at this point, he will reappear at the Burnt Tower after you've beaten the Elite Four.
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.
Every generation of Pokemon games has a legendary trio, from the legendary birds of Red and Blue, Articuno, Zapdos and Moltres, to the more recent psychic lake guardians Azelf, Mesprit and Uxie from Diamond and Pearl. Black and White introduces a dual Fighting-type trio, dubbed the musketeer trio, and Rock/Fighting-type Terrakion takes his place as one-third of that trio.
Pokemon Black / White – Terrakion waits for you within the depths of Victory Road once you've encountered Cobalion in Mistralton Cave.
Pokemon Black / White 2 – Terrakion is found on Route 22. If you return after defeating the Elite Four, it will be at a higher level.
Trivia: Terrakion is the only member of the musketeers with unique typing.