Arcanine, because of coarse, it's classified as the "Legendary Pokémon" and because of this:
When I first saw this in the episode I assumed that the 3 bird like Pokémon were the 3 legendary birds of Kanto, whilst the one on the bottom right I thought looked like Arcanine. This might be an Easter egg that they put in that maybe Arcanine was to be part of the trio to become a quad? That is a question for another topic, but it's part of the reason why I think Arcanine should be, and maybe was at one point, a legendary :)
When I first saw this in the episode I assumed that the 3 bird like Pokémon were the 3 legendary birds of Kanto, whilst the one on the bottom right I thought looked like Arcanine. This might be an Easter egg that they put in that maybe Arcanine was to be part of the trio to become a quad? That is a question for another topic, but it's part of the reason why I think Arcanine should be, and maybe was at one point, a legendary :)