Question

What makes a Pokemon legendary?

Answer 1

Aside from the high stats shared by most legendary Pokémon, many of them are only available once to the player in a given save file, and to obtain another legitimately, one must trade with another game. The gender of most legendary Pokémon is unknown (though there are seven notable exceptions in Latios, Latias, Heatran, Cresselia, Tornadus, Thundurus, and Landorus), and all but Manaphy are unable to breed in captivity, even with Ditto (and Manaphy only produces the unevolvable Phione when it breeds with Ditto).
No legendary Pokémon is known to evolve, though many are part of a legendary trio or legendary duo. Much like starter Pokémon appear at the beginning of each regional Pokédex and their generation's portion of the National Pokédex, legendary Pokémon typically appear at the very end of them. There is one exception: Victini. Victini's Unova Pokédex number is #000, coming before the region's starter Pokémon in the Pokédex.

-Bulbapedia

They also tend to have control over some part or process of the world, such as Suicune and the purification of water.

Comments

Very helpful.

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Just saying; I know you couldn't know this since this is posted in 2013 but there is a legendary that can evolve today, wich we all know as Nebby (Cosmog), that evolves into Cosmoem and then the mascots of Sun and Moon, Solgaleo and Lunala.

Answer 2

Well, what could you mean by that? If Game Freak make it legendary, it's legendary. xD

If you mean how to spot a legendary Pokemon, look out for

• Different Battle Music
• If it's interactable (you talk to it in overworld sprite, unless it's roaming)
• Cool legendary-like features like a special symbol on it's head for Suicune, and a long tail for Articuno, I guess (this doesn't apply to all Pokemon)

Okay, hope I helped. :)