Arcanine is it legendary?

Yes and No.

Arcanine was originally created as a legendary pokemon, but he was replaced by Moltres. This is why he is still categorized as 'Legendary' in the generation 1 pokedex. Because this was never fixed, Arcanine's species is 'Legendary'. This doesn't mean Arcanine is a legendary species but it's species is. Like Pikachu's species is mouse, Arcanine's species is Legendary.

But Arcanine is one of the strongest non legendary pokemon. His base stats are 555, and there is only one pokemon with higher base stats who isn't a legendary pokemon.