Hey, just one quick question. Is shaymin really considered a "legendary" pokemon?? The reason I'm wondering is because in the movie that shaymin is in, at the end of the movie their was obviously more than one, and I thought that in order for a pokemont to qualify as being "legendary" it had to be one of a kind.
No. In fact a number of legendary Pokemon have been confirmed NOT unique, in the anime and even the games. (According to the Pokedex an Entei is born every time a new volcano is, so there must be loads.)
I believe Nintendo has recognised the term 'legendary Pokemon' - they official stated Phione is legendary.
There is no trait, however, that definitively identifies a Pokemon as legendary. There always seem to be exceptions. Most can't breed, except Phione and Manaphy. Most have 580 or higher BST, except Phione. Most can only be got once in the game - but that goes for Rotom too. Anything with a BST above 600 is legendary - except Slaking.
Despite all that, the list of Legendaries is generally accepted to be the birds, Mewtwo, Mew, the beasts, Lugia, Ho-oh, Celebi, the golems, the super-ancient Pokemon, the Latis, Deoxys, Jirachi, the Lake Trio, the Dragon Trio, Heatran, Regigigas, Shaymin, Manaphy, Phione, Darkrai, Cresselia, and Arceus. The only ones that tend to get disputed were Phione prior to official confirmation, and Rotom who confused people by being a unique encounter with the legendary battle theme.