Question

Why is arcanine a legendary?

Answer 1

Any important belief is that Arcanine was going to be a fourth Pokémon to the Legendary birds' trio of Articuno, Zapdos and Moltres. It was present on an engraving shown in anime, episode - A Pokémon Emergency! Later a theory developed that Entei was created as Arcanine being a legendary couldn't come through.