Legendary Pokémon

Huh?

Ever since Pokémon Red & Green, the games have contained special one-of-a-kind Pokémon. These Pokémon are distinguished by their high stats, their powerful moves, the difficulty of catching them, and their inability to breed. Almost all of them have a special overworld sprite, and many of them are found sitting at the bottom of a "dungeon" or the top of a tower.

These Pokémon are also usually the subject of legends and stories in the Pokémon universe—and frequently feature in movies—which is why they are known as Legendary Pokémon.

But while Red & Blue only had 4 Legendary Pokémon—Articuno, Zapdos, Moltres, Mewtwo, and the elusive Mew—newer games contain ever-increasing numbers.

So here is a (hopefully) complete list of the Legendary Pokémon obtainable in each game, and how to get them, in reverse-chronological order.

Note: This document is incomplete. Bits that i am unsure of are marked with a "?". Some Pokémon and games are missing entirely.

Legend

Plot
This Pokémon must be fought or seen as part of the plot of the game. For these Pokémon, the requirements are usually not listed.
Event-only
This Pokémon cannot be encountered during the normal course of gameplay; it must be obtained or activated at a Nintendo-sanctioned giveaway.
Roaming
After being activated, this Pokémon will wander from Route to Route. If you are in the same location as it, you have a good chance of encountering it instead of a normal encounter. Roaming Pokémon will always try to flee the battle, unless Shadow Tag or Mean Look is in effect.

HeartGold/Soulilver

PokémonLevelLocationRequirements/Instructions
Lugia 45 Whirl Islands (SoulSilver only) Plot
70 Whirl Islands (HeartGold only) Beat the Champion. Obtain the Silver Wing from Mr. Pokemon(?).
Ho-oh 45 Bell Tower (HeartGold only) Plot
70 Bell Tower (SoulSilver only) Beat the Champion. Obtain the Rainbow Wing from Mr. Pokemon(?).
Raikou 40 Roaming, Burned Tower Plot
Entei 40 Roaming, Burned Tower Plot
Suicune 40 Route 25 Plot. Activate at the Burned Tower. Chase through Johto and Kanto to Route 25. Beat the Champion.
Articuno 50 Seafoam Islands
Zapdos 50 Route 10, outside the Power Plant Get all 16 badges?
Moltres 50 Mt. Silver Get all 16 badges.
Mewtwo 70 Cerulean Cave Get all 16 badges.
Latias 35 Roaming (HeartGold only) Obtain the national dex?
40 Pewter City (SoulSilver only) Event-only. Take the Enigma Stone to the Pewter City Museum and show it to Stephen Stone.
Latios 35 Roaming (SoulSilver only) Obtain the national dex?
40 Pewter City (HeartGold only) Event-only. Take the Enigma Stone to the Pewter City Museum and show it to Stephen Stone.
Kyogre ? Embedded Tower, on Route 47 (HeartGold only) Get all 16 badges and defeat Red. Get the Blue Orb(?) from Mr. Pokemon.
Groudon ? Embedded Tower, on Route 47 (SoulSilver only) Get all 16 badges and defeat Red. Get the Red Orb(?) from Mr. Pokemon.
Rayquaza ? Embedded Tower, on Route 47 Bring a Kyogre caught in HeartGold and a Groudon caught in SoulSilver to the Embedded Tower?
Dialga 1 Tanoby Ruins Event-only. Bring an Arceus to the Tanoby Ruins. Choose Dialga.
Palkia 1 Tanoby Ruins Event-only. Bring an Arceus to the Tanoby Ruins. Choose Palkia.
Giratina 1 Tanoby Ruins Event-only. Bring an Arceus to the Tanoby Ruins. Choose Giratina.

Diamond/Pearl

Roaming Pokémon can be tracked with the Marking Map on the Pokétch.

PokémonLevelLocationRequirements/Instructions
Uxie 50 Lake Acuity, near Snowpoint Plot
Mesprit 50 Roaming, Lake Verity, near Twinleaf Plot
Azelf 50 Lake Valor, near Pastoria Plot
Dialga 47 Spear Pillar (Diamond only) Plot
Palkia 47 Spear Pillar (Pearl only) Plot
Giratina 70 Turnback Cave, in Spring Path, off of Route 212 Obtain the national dex
Cresselia 50 Roaming, Fullmoon Isle, via Canalave Obtain the national dex. Talk to the ferryman in Canalave.
Darkrai 70? Newmoon Isle, via Canalave Event-only. Obtain the national dex. Bring the Member Card to the closed house in Canalave
Heatran 70 Stark Mountain
Regigigas 70 Snowpoint Temple Obtain the national dex. You must have Regirock, Regice, and Registeel in your party to awaken Regigigas.
Manaphy Beat Pokémon Ranger. Unlock and complete the Manaphy Egg event. Transfer the Manaphy Egg to Diamond/Pearl.
Shaymin 50? Flower Path? Event-only. Obtain the national dex. Bring Oak's Letter to Route 224.
Arceus 80 Hall of Origin, via Spear Pillar Event-only. Obtain the national dex. Bring the Azure Flute to the Hall of Origin

Platinum

Platinum contains the same Legendary Pokémon as Diamond/Pearl, with the following additions/changes.

Note: Starting in Platinum, Legendary Pokémon which are knocked out can be revived by beating the Elite Four again.

PokémonLevelLocationRequirements/Instructions
Origin Giratina 47 Distortion World Plot
Dialga 70 Spear Pillar Obtain the Adamant Orb and go to Spear Pillar?
Palkia 70 Spear Pillar Obtain the Lustrous Orb and go to Spear Pillar?
Articuno 50? Roaming Obtain the national dex. Talk to Professor Oak. (Anything else?)
Zapdos 50? Roaming Obtain the national dex. Talk to Professor Oak. (Anything else?)
Moltres 50? Roaming Obtain the national dex. Talk to Professor Oak. (Anything else?)
Regigigas 1 Snowpoint Temple Obtain the national dex. You must have Regirock, Regice, and Registeel in your party to awaken Regigigas.
Regirock ? ? Event-only. Bring an event Regigigas somewhere.
Regice ? ? Event-only. Bring an event Regigigas somewhere.
Registeel ? ? Event-only. Bring an event Regigigas somewhere.

XD

PokémonLevelLocationRequirements/Instructions
Lugia ? Plot. Purify Shadow Lugia.

FireRed/LeafGreen

Roaming Pokémon can be tracked via the map in the Pokédex. If a battle with a roaming Pokémon is ended with Roar or Whirlwind, it will disappear forever.

PokémonLevelLocationRequirements/Instructions
Raikou 50 Roaming Obtain the national dex? Appears if you chose Squirtle as your starter.
Entei 50 Roaming Obtain the national dex? Appears if you chose Bulbasaur as your starter.
Suicune 50 Roaming Obtain the national dex? Appears if you chose Charmander as your starter.
Mewtwo 70 Cerulean Cave Beat the Champion.
Deoxys 30? Birth Island Event-only. Show the Aurora Ticket to the boatman. Push the triangle around correctly.
Lugia 70? Navel Rock, right path Event-only. Show the Mystic Ticket to the boatman.
Ho-oh 70? Navel Rock, left path Event-only. Show the Mystic Ticket to the boatman.

Colosseum

PokémonLevelLocationRequirements/Instructions
Ho-oh ? Purify all shadow Pokémon and complete Mt. Battle.

Ruby/Sapphire

PokémonLevelLocationRequirements/Instructions
Kyogre 45 Cave of Origin (Sapphire only) Plot.
Groudon 45 Cave of Origin (Ruby only) Plot.
Rayquaza 70 Sky Pillar Beat the Champion.
Regirock 40 Desert Ruins, on Route 111 Solve the puzzle in the Sealed Chamber under Route 134. Solve the puzzle in the Desert Ruins.
Regice 40 Island Cave, on Route 105 Solve the puzzle in the Sealed Chamber under Route 134. Solve the puzzle in the Island Cave.
Registeel 40 Ancient Tomb, on Route 120 Solve the puzzle in the Sealed Chamber under Route 134. Solve the puzzle in the Ancient Tomb.
Latias 40 Roaming (Sapphire only) Beat the Champion.
40? Eon Island (Ruby only) Event-only. Show the Eon Ticket to the Captain Stern(?).
Latios 40 Roaming (Ruby only) Beat the Champion.
40? Eon Island (Sapphire only) Event-only. Show the Eon Ticket to the Captain Stern(?).
Jirachi Event-only. Transfer from the Pokémon Colosseum bonus disc.
Deoxys Event-only

Emerald

Emerald adds the two event-only islands from FireRed/LeafGreen, plus a third island.

PokémonLevelLocationRequirements/Instructions
Rayquaza 70? Sky Pillar Plot.
Kyogre 70? ? Something about the Weather Institute.
Groudon 70? ? Something about the Weather Institute.
Latias 40 Roaming Beat the Champion. Tell your mother you saw a red Pokémon on television.
40? Eon Island Event-only. Show the Eon Ticket to the Captain Stern(?). Appears if you chose Latios.
Latios 40 Roaming Beat the Champion. Tell your mother you saw a blue Pokémon on television.
40? Eon Island Event-only. Show the Eon Ticket to the Captain Stern(?). Appears if you chose Latias.
Deoxys 30? Birth Island Event-only. Show the Aurora Ticket to ?. Push the triangle around correctly.
Lugia 70? Navel Rock, right path Event-only. Show the Mystic Ticket to ?.
Ho-oh 70? Navel Rock, left path Event-only. Show the Mystic Ticket to ?.
Mew ? Faraway Island Event-only. Show the Old Sea Map to ?. Play hide-and-seek with Mew.

Red/Blue

PokémonLevelLocationRequirements/Instructions
Articuno 50? Bottom floor of Seafoam Islands
Zapdos 50? Power Plant
Moltres 50? End of Victory Road
Mewtwo 70 Bottom of Cerulean Cave Beat the Champion
Mew Event-only