Pokemon #666 may be some comical, "boon" Pokemon like Delibird or Farfetch'd. The Pokemon franchise has never been centered around any religion or even given a nod towards any of the current religious archetypes (Christianity, Buddhism, etc.). Yes, the legendary Pokemon of each region starting with Generation 2 are themed after certain mythologies but never any serious religions. Generation 4 is an exception as it was centered around the idea of creationism from the God Particle (research it).
Anyway, back to the subject, no, I don't think #666 will be any seriously demonic Pokemon. If anything, it may be an imp-like Pokemon, or even a Sableye evolution, but that'd be as far as they take it.