>>51044787Vulpix/Ninetales are based on red foxes & kyuubi folklore; Fennekin(et al.) are based on fennec foxes which live in the desert. It makes sense for them to both be fire type, particularly in Fennekin's case. To further differentiate them, one line is quadruped exclusively while the other line is part bipedal. There's enough differences to make them both distinct.
Also the more foxes the better as far as I'm concerned.