Legendary trio

A legendary trio is a term used to describe a group of three closely-related legendary Pokémon. So far, this has described five groups of Pokémon: the Kanto legendary birds, Johto's legendary beasts, Hoenn's legendary golems, Sinnoh's lake trio, and Hoenn's trio of weather-related Pokémon, Groudon, Kyogre, and Rayquaza.

Though far less common, some Pokémon appear in legendary duos, such as Mewtwo and Mew, Lugia and Ho-Oh, Latios and Latias, and Dialga and Palkia. Due to their being one short of the now-standard trio, many theories have involved a hypothetical third member of these duos, most recently Dialga and Palkia's third counterpart, which is theorized would be the Pokémon representing the third version of Diamond and Pearl, much as Rayquaza represented the third version of Ruby and Sapphire. Much speculation has been towards the Pokémon Giratina taking that role, due to its similar typing to the other two, being half Dragon-type like both Dialga and Palkia. Some speculation also notes Arceus, who is said to be the creator of the Space-Ruling and Time-Ruling Pokémon. Either way, no official evidence supports a third member of this duo, and so only time will tell.