I just thought of something, from what I heard, Mew is a genetically unstable Pokemon, so is Eevee, Eevee is like mew, a much less drastic scale of course but still...... My theory is that mew and Eevee were created to be equals, but Eevee turned out to be more genetically stable than Mew did therefore leading to Eeveeloution. This is just a theory, but just think about it.......