Legendary Pokémon are very rare Pokémon in the Pokémon world. Characters in the series often claim that very few people have seen them. They are hard to catch, and are stronger than most other Pokémon. They have powers that other Pokémon do not have. In the games some roam around, while others stay in one spot until the trainer knocks it out or captures it. If the player defeats a legendary Pokémon, they will often not be available again, because there is only one of them, in rare cases they might reappear after certain conditions are met. Because of this, players usually save their game before going to battle the Pokémon. The Master Ball and Ultra Ball are popular choices for catching legendary Pokémon. But legendary Pokémon can also be caught with a normal Poké Ball. It is often advised to catch a legendary Pokémon with a Quick Ball since a Quick Ball has a high capture rate at the beginning of a battle. Legendary Pokémon typically have a 3/255 catch rate with Ultra Balls, and 1/255 catch rate with regular Poké Balls. There are about 900 Pokémon.

Kanto
These Pokémon are in the Kanto region, which appears in Pokémon Red and Blue, and its related games.
  - An Ice/Flying-type Pokémon. It is the first Legendary Bird. Articuno is in the Seafoam Islands of Kanto. It is also one of the shadow Pokémon in Pokémon XD: Gale of Darkness used by Greevil. It is also seen flying around Sinnoh in Pokémon Platinum after the player talks to Prof. Oak, and in Kalos in Pokémon X and Y if the player chose Chespin. When released from a Poké Ball in Super Smash Bros. Melee, Articuno will use a Blizzard attack that freezes any opponent.
  - An Electric/Flying-type Pokémon. It is the second Legendary Bird. Zapdos is in the Power Plant in Kanto. It is also one of the shadow Pokémon in Pokémon XD: Gale of Darkness used by Greevil. It is also seen flying around Sinnoh in Pokémon Platinum after the player talks to Prof. Oak, and in Kalos in Pokémon X and Y if the player chose Fennekin. When released from a Poké Ball in Super Smash Bros. Melee, Zapdos uses a powerful Thunder Shock that attacks several times.
  - A Fire/Flying-type Pokémon. It is the third Legendary Bird. Moltres is in the Victory Road in early games. In FireRed and LeafGreen, Moltres is in Mount Ember on the first Sevii Island. It is also one of the shadow Pokémon in Pokémon XD: Gale of Darkness used by Greevil. It is also seen flying around Sinnoh in Pokémon Platinum after the player talks to Prof. Oak, and in Kalos in Pokémon X and Y if the player chose Froakie. In Pokémon HeartGold and SoulSilver it is in Mt. Silver. In Super Smash Bros. Melee or Brawl when released from a Poké Ball, Moltres will flap its wings, burning players. It will then fly away.
  A Psychic-type Pokémon. Players can get Mewtwo in the Cerulean Cave in Kanto (except in games based on Gold and Silver) or in the Unknown Dungeon in Kalos, both of which open after defeating the Elite Four. Mewtwo is a character in Super Smash Bros. Melee, and Super Smash bros 4 and uses the moves Shadow Ball, Confusion, Teleport, and Disable. In Pokémon X and Y, Mewtwo gained the ability to Mega Evolve into two different forms: Mega Mewtwo X, which is more physically based than normal Mewtwo, and Mega Mewtwo Y, which has the highest Special Attack of any Pokémon,
  - A Psychic-type Pokémon. Mew can only be obtained through special events or by using a cheating device like a Gameshark. In the Super Smash Bros. series, when released from a Poké Ball, Mew will give the players bonus points, and fly away. In Brawl it can drop a CD.

Johto
These Pokémon are in the Johto region, which appears in Pokémon Gold and Silver, and its related games.
  is a Electric-type Pokémon. It roams after the player makes them leave from Ecruteak City's Burned Tower. It appears in Kanto in Pokémon FireRed and LeafGreen if the player started off with Squirtle, but, due to an error with roaming wild Pokémon in these games, will have lower stats than other Pokémon. When released from a Poké Ball in Super Smash Bros. Melee, Raikou releases a powerful Spark attack that shocks any opponents that get caught in it.
  is a Fire-type Pokémon. It roams after the player makes them leave from Ecruteak City's Burned Tower. It appears in Kanto in Pokémon FireRed and LeafGreen if the player started off with Bulbasaur, but, due to an error with roaming wild Pokémon in these games, will have lower stats than other Pokémon. When released from a Poké Ball in Super Smash Bros. Melee or Brawl, Entei unleashes a volcanic Fire Spin that deals a lot of damage to any opponents that get trapped in it.
  is a Water-type Pokémon. It roams after the player makes them leave from Ecruteak City's Burned Tower. In Pokémon Crystal it goes to the Tin Tower instead of roaming. It appears in Kanto in FireRed and LeafGreen if the player started off with Charmander but, due to an error with roaming wild Pokémon in these games, will have lower stats than other Pokémon. When released from a Poké Ball in Super Smash Bros. Melee, Suicune unleashes a blast of Icy Wind that causes high levels of damage. Suicune returns in Brawl but with a different attack, Aurora Beam.
 Lugia is a Psychic/Flying-type Pokémon, and is the mascot of Pokémon Silver. It also appears in Super Smash Bros. Brawl and Wii U as a Pokeball. It the Pokémon represented by the silver wing which is shown in Johto.
 Ho-Oh is a Fire/Flying-type Pokémon that is said to grant a person it's greatest desire by flying over the person. Ho-oh has flown over Ash Ketchem before, some say this is why he does not age in the series. This Pokémon is the mascot of Pokémon Gold. It also appeared in the first episode of the anime.
 Celebi

Hoenn
These Pokémon are in the Hoenn region, which appears in Pokémon Ruby and Sapphire, and its related games.
 Regirock
 Regice
 Registeel
 Latias is a Dragon/Psychic type Pokémon. In most games Sapphire, as a Roaming Pokémon (In Ruby, Latios will appear instead, but an event item allows you to go to an island where it is catchable. Also, there is an error in Sapphire that causes Roaming Pokémon to have lower stats than other Pokémon), Emerald (You get a choice after the Elite Four. The error was fixed in Emerald.), HeartGold (Latios appears in SoulSilver), and White 2 (Latios appears in Black 2). It has its own special item, which it shares with its counterpart. It also obtained a Mega Evolution in X and Y, but there was no way to obtain the Mega Stone needed until the next set of games. Latias has higher defensive stats than its counterpart.
 Latios is a Dragon/Psychic type Pokémon. In most games (Ruby, as a Roaming Pokémon (In Sapphire, Latias will appear instead, but an event item allows you to go to an island where it is catchable. Also, there is an error in Ruby that causes Roaming Pokémon to have lower stats than other Pokémon), Emerald (You get a choice after the Elite Four. The error was fixed in Emerald.), SoulSilver (Latias appears in HeartGold), and Black 2 (Latias appears in White 2). It has its own special item, which it shares with its counterpart. It also obtained a Mega Evolution in X and Y, but there was no way to obtain the Mega Stone needed until the next set of games. Latios has higher offensive stats than its counterpart.
  is a Water-type Pokémon. It is in the Cave of Origin in Pokémon Sapphire, and Marine Cave in Pokémon Emerald. In Pokémon HeartGold Kyogre is at the Hidden Tower after the player gets the Blue Orb. After being released from a Poké Ball in Super Smash Bros. Brawl, Kyogre opens its mouth and squirts out a stream of water. The water is non-damaging, but causes characters to be knocked back with lots of force. Touching Kyogre's body will do damage. Kyogre also appears as a trophy.
  is a Ground-type Pokémon. It is in the Cave of Origin in Pokémon Ruby, and Terra Cave in Pokémon Emerald. In Pokémon SoulSilver Groudon is at the Hidden Tower after the player gets the Red Orb. After being released from a Poké Ball in Super Smash Bros. Brawl, it uses Overheat causing anyone to come into contact with it to be set on fire and sent flying. Due to its huge size, it takes up a lot of the stage, making its attack hard to avoid.
  is a Dragon/Flying-type Pokémon. In Pokémon Ruby, Sapphire, and Emerald, it is in Sky Pillar. In Pokémon HeartGold and SoulSilver Rayquaza is at the Hidden Tower after the player gets the Green Orb from Professor Oak, by showing him a Groudon and Kyogre that were obtained in HeartGold and SoulSilver. Rayquaza shows up in Super Smash Bros. Brawl as a boss in the game's story mode.
 Jirachi
 Deoxys

Sinnoh
These Pokémon are in the Sinnoh region, which appears in Pokémon Diamond and Pearl, and its related games.
 Uxie
 Mesprit
 Azelf
  is a Steel/Dragon-type Pokémon. Dialga appears as a stage Pokémon in Super Smash Bros. Brawl. It affects the stage by slowing down time, and even going off of its post to attack the players in the middle. It can also distort the stage, as well as shoot beams.
  is a Water/Dragon-type Pokémon. Palkia appears as a stage Pokémon in Super Smash Bros. Brawl. It uses its powers to distort the stage however possible. Some examples include tilting the stage, turning the stage completely upside down and reversing the screen itself (the last two will cause the movement controls to be reversed temporarily). It often shoot beams in the cave below, dealing a lot of damage to the players that get caught in the blast.
 Heatran
 Regigigas
 Giratina is a Ghost/Dragon type Pokémon with two forms- an Altered Form, which is its form in most cases, and has higher defensive statistics, and an Origin Form, which is its form if it is in the Distortion World, or holding a certain item. Due to this item and the Distortion World not existing in Pokémon Diamond and Pearl, a Giratina in those games can only be in Altered Form.
 Cresselia
 Phione
 Manaphy
 Darkrai is a dark type Pokémon. In another language, rai means the word dark. Darkrai could either mean dark dark or dark cry. Either way, it's name is not welcoming.
 Shaymin
 Arceus is known as the God of Pokémon. In Pokémon Diamond, Pearl, and Platinum, it can only be awakened through a special event where the player receives an Azure Flute, which was never released in any area of the world. The player then enters the Hall of Origin, battles Arceus and can try to catch it. Arceus has also been distributed multiple times, and there is an event in Pokémon HeartGold and SoulSilver that involves an Arceus received through one of these events, even though the Azure Flute is cheating-only. Arceus has the highest base-stat total (720) of all non-Mega Evolved Pokémon.

Unova
These Pokémon are in the Unova region, which appears in Pokémon Black and White and Pokémon Black 2 and White 2.
 Victini is the first Fire/Psychic-type Pokémon known as the Victory Pokémon. It is a small rodent-like Pokémon with large pointed ears shaped like letter V. Victini was designed to be more appealing to women. Victini can be obtained from a wi-fi event. For the first month of the game's release, the Liberty Ticket can be obtained, which allows the player to find Victini.
  is a Steel/Fighting-type Pokémon known as the Iron Will Pokémon. It bears a close resemblance to a markhor. After getting Cobalion in Mistralton Cave, Terrakion and Virizion are able to be fought.
  is a Rock/Fighting-type Pokémon known as the Cavern Pokémon. It resembles an ox. It is at Victory Road after battling Cobalion.
  is a Grass/Fighting-type Pokémon known as the Grassland Pokémon. It resembles an antelope. It is at Pinwheel Forest after battling Cobalion.
  is a Flying-type Pokémon known as the Whirlwind Pokémon. He is based on Fūjin, the god of wind. Tornadus can only be encountered in Pokémon Black.
  is a Electric/Flying-type Pokémon known as the Lightning Strike Pokémon. He is based on Raijin, the god of thunder and lightning. Thundurus can only be encountered in Pokémon White.
  is a Dragon/Fire-type Pokémon known as the Vast White Pokémon, and is the mascot for Pokémon Black. It resembles a white wyvern, with wing-shaped arms and claws.
  is a Dragon/Electric-type Pokémon known as the Deep Black Pokémon, and is the mascot for Pokémon White. It resembles a black dragon/dinosaur, with wing-like formations on its forearms.
  is a Ground/Flying-type Pokémon known as the Abundance Pokémon. He is based on Inari, the god of fertility. Landorus can only be encountered at a location known as the Abundant Shrine if the player has both Tornadus and Thundurus in their party.
  is a Dragon/Ice-type Pokémon known as  the Boundary Pokémon. It resembles a large bipedal gray dragon with various icy appendages. Kyurem has been shown in two forms, Black Kyurem and White Kyurem.
 Keldeo is a Water/Fighting-type.
 Meloetta is a Normal/Psychic Pokémon known as the melody Pokémon. This Pokémon holds the key to the reveal glass, which is able to transform Tornadus, Thundurus and Landorus into their Therian forms. It also has a Pirouette form.
 Genesect is a bug/steel Pokémon and is obtained by a WiFi event. There is an explanation of it in Japanese.

Kalos
These Pokémon are in the Kalos region, which appears in Pokémon X and Y.
  is a Fairy-type Pokémon known as the Life Pokémon. It is the mascot for Pokémon X. It resembles a deer, with X-shaped glints in its eyes and jewels on its antlers.
  is a Dark/Flying-type Pokémon known as the Destruction Pokémon. It is the mascot for Pokémon Y. It resembles the letter Y.
  is a Dragon/Ground-type Pokémon known as the Order Pokémon. Its body resembles the letter Z.
 Diancie is a Rock/Fairy Pokémon known as the jewel Pokémon.
 Hoopa is a Ghost/Psychic Pokémon known as the mischief Pokémon.
 Volcanion a Fire/Water Pokémon known as the steam Pokémon.

Alola
These Pokémon are in the Alola region, which appears in Pokémon Sun, Moon, Ultra Sun and Ultra Moon Type: Null
 Silvally
 Tapu Koko
 Tapu Lele
 Tapu Bulu
 Tapu Fini
 Cosmog (コスモッグ Causesmog) is a Psychic-Type Pokémon known as the Nebula Pokémon. Cosmog is a small Pokémon with a gaseous body similar to a nebula. It evolves into Cosmoem starting at level 43, which evolves into either Solgaleo or Lunala  starting at level 53 depending on the game it evolves in.
 Cosmoem (コスモウム Causemoem) is a Psychic-Type Pokémon. Cosmoem is a Pokémon in the shape of a translucent, dark blue orb with a starscape pattern set in a gold-colored structure, which is made up of four ellipses intersecting each other in an asterisk shape and another intersecting the former four along the middle. It evolves from Cosmog starting at level 43 and evolves into either Solgaleo or Lunala  starting at level 53 depending on the game it evolves in.
 Solgaleo (ソルガレオ, sulgaileo) is a Psychic/Steel-type Pokémon known as the Sunne Pokémon. It is the mascot for Pokémon Sun and Pokémon Ultra Sun. Solgaleo is a large, white Pokémon resembling a white lion. It evolves from Cosmoem when leveled up in Pokémon Sun starting at level 53. It is one of Cosmog's final forms, the other being Lunala. Solgaleo is a member of the cosmic duo with Lunala.
 Lunala (ルナアーラ, Lunnala'') is Psychic/Ghost-type Pokémon known as the Moone Pokémon. It is the mascot for Pokémon Moon and Pokémon Ultra Moon. Lunala is a large Pokémon resembling a skeletal, legless bat. It evolves from Cosmoem when leveled up in Pokémon Moon starting at level 53. It is one of Cosmog's final forms, the other being Solgaleo. Lunala is a member of the cosmic duo with Solgaleo.
 Necrozma is a Psychic-Type Pokémon known as the Prism Pokémon. It is the mascot of Pokémon Ultra Sun (With Solgaleo) and Pokémon Ultra Moon (With Lunala). Necrozma is a Pokémon whose body appears to be constructed out of black crystalline material; its disproportionately large arms have three claws jutting out of them, as well as a single large whitish crystal protruding backwards from each arm. Its small legs bend backwards like a bird's and have three toes each. Although it is not known to evolve, it has three other forms.
 Dusk Mane Necrozma is a Psychic/Steel-type Pokémon. It is a fusion from Solgaleo and Necrozma.
 Dawn Wings Necrozma is a Psychic/Steel-type Pokémon. It is a fusion from Lunala and Necrozma.
 Ultra Necrozma is a Psychic/Dragon-type Pokémon.
 Magearna
 Marshadow
 Zeraora

References

Pokémon monsters