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It is a legend as in like a kracken, but it is not a legendary pokemon. By that I mean that arcanine has legends written about it but is not a legendary as such.
growilths are relatively easy to catch, and Growlith>arcanine... sooo.... no
that'd be like saying bidoof was worth anything other than the title HM Slave... it doesn't work
Erm as much as I love Arcanine, and it's glory, I don't consider it a Legendary. Although it's Pokedex does says so, and it was shown on a tablet with Articuno, Moltres, and Zapdos I still don't think that it's a legendary. (ARCANINE POWER!!!!)