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I fucking hope not.
Arcanine is one of my all time favorites. So are Heracross and Gengar, and both of their Megas are absolutely abysmal. In fact, their Megas are the ONLY two I don't like. And by "don't like", I mean they're so goddamn hideous that I pretend they don't exist.
Due to this, I can harbor nothing but fear and hatred toward potential Megas for my favorites. I only want Pokémon that I don't care about to get Megas, because every time one I love gets one, its stupid looking.
tl;dr - If Arcanine gets a mega they'll totally botch it.