666 is a Christian thing, Game Freak is based in Japan which basically views foreign religions as fantasy in the same way we view wizards and dragons. Unlikely they'll do anything special for 666. --- http://i.imgur.com/FAoMm.png
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658 will be the last evolution of the last starter (probably water). I'd say 659-680 or so will be the early game stuff such as the generic normal mammal, the obligatory normal/flying and bug/flying types, and maybe the Pikachu knockoff.
Based on the last two gens, the starter Pokemon after the starts went 3 stage (Starly, Lillipup), 2 stage (Bidoof, Patrat), and 2 stage (Kriketot, Kriketune). What came following those are the Shinx line and the elemental monkeys, neither of which really have anything in common except early game mons. What would 666 would be a first evolution based on that trend.
Yes, I know Gamefreak doesn't sit down and say "we need a three stage following the starters in the Pokedex." Just an observation.
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