The Power of Three said:
Since when is there more than one mew? Mewtwo doesn't count.
Since when can there be more than one of those two?
Everyone seems to believe, that for some reason, there is only one of each legendary because they are "legendary".
the only reason for them being called "legendary" is because a long time ago, one of those species did something legendary, or was included in a legend of old, but there are just as many of those Pokémon as any other. There's visual proof in the anime during the Whirl Islands episodes that there are more than one Lugia. During the Pokémon League episodes "A Moltres" was mentioned instead of "THE Moltres". During Pokémon 4Ever, multiple Celebi were seen, and they also mention "A Suicune" instead of "THE Suicune". In Pokemon Heroes (5th movie) multiple Latios and Latias were seen. During the Pokémon Ranger movie, it was made known there are multiple Manaphy's and Phione's. And as the Pokémon Frontier episodes show, there are clearly more than one of each of the Regi's and Bird Trio's. During the 7th movie, "A Rayquaza" was mentioned instead of "THE Rayquaza" and obviously, there's more than one Deoxys as well.
The only pokemon that weren't proven to have more than one are the main legendaries from the 4th generation, Mew, and Mewtwo, however I'm pretty sure that the 3 Mirages have multiple species as well.
You should also know that Arcanine and Ninetales are official legendary pokemon, as mentioned in several episodes and in-game Pokedex entries.