As Mew said it is not Legendary in pokemon terms of what a legendary is. Especially in the anime when you have "Legendary" pokemon in the game that are common(not super common) like the regis, Heatran, the bird trio, Shaymin. According to Bulbapedia he is based off a Korean mythical creature Xiezhi which resembles a lion but is a fire dog. Its probably no mistake that Arcanine was released as an event pokemon in 2009 Battle Series tournament, which was Korean exclusive. I don't know about Arcanine being one of the strongest Fire types since we have seen plenty of Ash Charizard and pokes like Harrison Blaziken. And Phione is a pre evo of Manaphy, although it cannot directly evolve into Manaphy.