It doesn't ring any bells for me SonicTH, but i'll search around for you.
No, Arcanine is not a Legendery. That term is just used in the PokéDex to describe it's strength and species - but that can just be thought as a matter of opinion. Anyway, it can't be because Legendaries cannot evolve.
I found an artical on it from Wikipedia; (although this isn't a trustworthy source I do agree it somewhat makes sense.)
In the Series 1 episode: "Emergency!" An image of Arcanine appears on a stone tablet of Legendary Pokémon, alongside Articuno, Zapdos and Moltres.