Legendary Pokémon as the name suggests, are a group of incredibly rare, and often very powerful Pokémon. They can also be Pokémon illusions. They are very rare and powerful Pokémon, most being unique to only one region. Legendary Pokémon can also be seen as mythological, such as the Sinnoh Legendaries that shaped the world. Legendary Pokémon can only be found once per game file. In the show it is assumed that there are more than one of each legendary throughout the Pokémon World. They are easy to beat at a high level but hard to catch.
While all are typically grouped under the banner of "legendary", there are several distinct terms that are used to refer to them.
Mirage PokémonMirage Pokémon, refers to pokémon seen so rarely, that some question their very existence.
Super Ancient PokémonSuper Ancient Pokémon, refers to pokémon that have dealing mostly with weather. The best examples of this type of pokémon is the Weather Trio of Sinnoh Groudon, Kyogre and Rayquaza or the Kami Trio of Unova Tornadus, Thundurus, and Landorus.
Mystical PokémonMystical Pokémon usually refer's to pokemon dealing with some sort of balane such as Yin and Yang. The Tao Trio of the Unova Region are the best example's of these type's of pokémon.
Deity PokémonDeity Pokémon are pokémon refered to as god-like pokémon. Arceus of Sinnoh for example is said to have created the universe with it's 1000 arm's giving it a god-like title. Deity Pokémon in truth aren't god's as they are not immortal and they can be killed just like normal pokémon.
Ancient Titan Pokémon
Ancient Titan Pokémon are powerful pokémon said to have exsisted before man walked the earth. These pokémon like Deity Pokémon aren't god's, but they are very powerful.
Kamisama Pokémon are god's among the pokémon species. These pokémon are immortal by nature and are the most powerful of pokémon on the planet. These pokémon are rarly seen or ever metioned as few know of there exsitance.