Legendary is just anything with 600 BST and above and difficult to obtain. 600BST but easy to obtained Pokemon are pseudo-legendaries. The 580 BST legendaries are called pseudo-non-legendary-legendaries.
Anyway, there's probably at least urban legends about a strong monster created by humans that it's roaming free.
It's true. A legendary is stated as any Pokemon with a Base Stat Total of 600 or more, cannot evolve or breed and are only available once from each save file without trades.
There is no specific criteria for Legendaries. Fans have been trying to come with rules that define a legendary for years and they're proven wrong every new generation. Before Gen 3, the "rules" stated that legendary Pokemon cannot have genders. Here come Latias and Latios. Now we know legendaries cannot hatch from eggs, and they can't have a 50/50 chance of both genders. Whoops, here's Manaphy and Heatran. But at least now we know legendaries are always at the end of the Pokedex. Oh wait no, here's Victini.
Face it, the so-called "rules" change every generation. So it's futile to try and make up some list of criteria for determining a legendary. The only thing that makes a legendary a legendary is Game Freak's word. In Gen 6 there could be a Pokemon that has 300 BST, breeds, evolves, has multiple copies in the same game and is in the middle of the Pokedex and if GF says it's a Legendary then it's a Legendary. We already know who is legendary and who isn't. The only exception to this is Phione, because official sources contradict each other. Gen 1 has 5, Gen 2 has 6, Gen 3 has 10, Gen 4 has 13 (14 counting Phione), Gen 5 has 13, and Gen 6 has 2 that we know of so far. That's it. Contrary to popular belief, Unown, Arcanine, Volcarona, and Rotom are not legendaries, and there is no official list of rules for determining a legendary. Fans can try to make up lists of rules all they want, but they're always proven wrong come next generation.
Your opinion differs from mine, therefore you're trolling.