A Legendary duo is a pair of closely related Legendary and Mythical Pokémon, either representing teamwork or opposition.
In the games, Legendary duos are often found after the player enters the Hall of Fame but they usually are not as important to the main storyline as the trios are. Duos or their component parts are featured in several of the movies as keepers of peace.
Sometimes, pairs of Legendary Pokémon once only presented as duos are revealed to actually be Legendary trios in a subsequent game or games of the same generation. However, even within trios, the dynamic between two of the members is often very close to that of a duo: Kyogre and Groudon, Dialga and Palkia, Reshiram and Zekrom, Xerneas and Yveltal, Solgaleo and Lunala are all examples of this.
These duos are pairs of Pokémon with similar stats and movesets, as well as a relationship in lore; the members are usually presented as equals, and function in a symmetrical dynamic.
The tower duo is a fan term for the Legendary duo of Johto that consists of Lugia and Ho-Oh. Lugia and Ho-Oh are often regarded as polar opposites. Lugia represents the sea, storms, the color silver and is the mascot of Pokémon Silver and Pokémon SoulSilver, while Ho-Oh represents the sky, rainbows, the color gold, and is the mascot of Pokémon Gold and Pokémon HeartGold. Both are also trio masters: Lugia is the master of the Legendary birds, while Ho-Oh is the master of the Legendary beasts.
Their legend is introduced in Generation II, and states that the two birds each resided on a tower: Lugia on the Brass Tower, and Ho-Oh on the Bell Tower. When the Brass Tower was destroyed (prior to the start of the second generation games), the pair was split up, and Ho-Oh created the Legendary beasts: Entei, Raikou, and Suicune.
The eon duo is a fan term for a Legendary duo of Hoenn that consists of Latias and Latios, which are also that region's roaming Pokémon. The two share their type combinations, two of their base stats and even several moves they learn when they level up. Both were featured in the movie Pokémon Heroes: Latios & Latias, where they were revealed to be playful siblings.
Latios and Latias take on counterpart availability in Pokémon Ruby, Sapphire, and Emerald, and the remake games Omega Ruby and Alpha Sapphire, as well as Pokémon HeartGold and SoulSilver and Black 2 and White 2, with Latios being available in Ruby, SoulSilver, Black 2, and Omega Ruby, and Latias being available in Sapphire, HeartGold, White 2, and Alpha Sapphire. In Emerald, after the player has defeated the Elite Four, their mother will ask if the Pokémon mentioned on TV was red or blue, and depending on that, one of the two will be roaming Hoenn. The other member of the duo will be found on Southern Island, accessible by Eon Ticket, in Ruby, Sapphire, Emerald, Omega Ruby, and Alpha Sapphire, and in Pewter City, through the Enigma Stone event, in HeartGold and SoulSilver.
The lunar duo is a fan term for a duo of Sinnoh that consists of the LegendaryCresselia and the MythicalDarkrai. Cresselia and Darkrai are commonly seen as polar opposites, just like the aforementioned Tower duo of Lugia and Ho-Oh. Also, Cresselia and Darkrai reside on Fullmoon Island and Newmoon Island respectively, which are direct mirror images of each other. Cresselia and Darkrai are also known as the subconscious duo because of how Darkrai is said to give people nightmares, while Cresselia does the opposite (giving people pleasant dreams). Furthermore, Cresselia and Darkrai also appear in Pokémon Mystery Dungeon: Explorers of Time, Explorers of Darkness and Explorers of Sky as enemies.
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Zacian and Zamazenta were introduced in Generation VIII, acting as the guardians of the Galar region during the Darkest Day. After Eternatus's defeat 3,000 years before the start of Sword and Shield, the duo retreated to the Slumbering Weald, whereupon they assumed the form of statues. Their roles in the Darkest Day would come to be covered up by the kings of Galar and the two have since faded into obscurity by the time of the game's start. Zacian is sometimes described as Zamazenta's older sister, hinting at a sibling relationship between the two. Zacian specializes in offense while Zamazenta has great defense. In their base forms, the two have identical stats; their Crowned Forms have unique stats, but the same base stat totals.
These duos are made up of Pokémon with a strong relation in the lore, but their stats and movesets tend to be fairly distinct and not equivalent; these duos can be said to be asymmetrical compared to the true duos.
Mewtwo and Mew make up Kanto’s Legendary duo. They are both very elusive, if not unique, Psychic-type Pokémon. Mewtwo was cloned from Mew, meaning the two share DNA and many traits. In the games, however, they are usually treated as unrelated Pokémon, with Mewtwo's stats, level up moves, and treatment in the Pokémon Stadium series being closer to that of the tower duo, while Mew's are closer to that of Celebi.
In Mewtwo Strikes Back, Mew embodied innocence and joy, but Mewtwo's experiences as it gained consciousness gave it a jaded and cynical view of humanity. The duality expressed between the two characters formed the basis of their conflict, but innocent Mew, with the help of Ash Ketchum, was able to sway Mewtwo's contempt.
Manaphy and Phione are so far the only two Mythical Pokémon capable of breeding. Each can only breed with Ditto, and Eggs from both Pokémon will produce only Phione. The sea guardian duo has many similarities to the Mew duo; instead of an unnatural (through genetic engineering in a lab), stronger version, Phione is a natural (through breeding), weaker version of Manaphy; both also appear before the "original" in the Pokédex.
Due to conflicting evidence from official sources, Phione's status as a Mythical Pokémon is uncertain; if it is not a Mythical Pokémon, the sea guardian duo is not a Legendary duo.
