A Legendary or Mythical group is a group of closely related Legendary or Mythical Pokémon. Legendary Pokémon often come in groups. Some Pokémon are part of more than one group, and most groups have a master.
The three Legendary birds, also known as the Winged Mirages, are the resident Legendary trio of the Kanto region. Each of them is based on a mythical bird, and each is associated with a season (though none of the three is associated with autumn).
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.
Like the Legendary birds of the previous generation, a second trio of similar types was introduced in the neighboring Johto region. These Legendary beasts, so named because their features are based on many different creatures, both real and mythical, as well as mythological deities, were trapped in the Brass Tower when it was struck by lightning and burned to the ground, resulting in their death. The trio was revived by the power of Ho-Oh and now roam across the land because of their great power. It is unknown whether the three were already a Suicune, Raikou, and Entei when they died, or whether they were simply three non-Legendary Pokémon, and Ho-Oh reincarnated them as the first Legendary beasts. In the case of the latter, it is possible that these three would be the original three beasts, who created multiple descendants. For example, a new Entei is said to be born whenever a new volcano is formed.