I don't think there will be anything 'special' for #666. It's a christian concept and unknown to Japanese.
Also, current national dex ends at 649. We can add 9 starters to go to 658. i expect Regional rodent to be 659 and 660. Next can be another evolutionary line ( ala lillipup , Kricketot and poocheyena).
Thus, according to me, #666 can belong to the regional Bird evolutionary line. But they have interchanged the sequence of Rodent and Bird in past generations. so it can also be the Regional Rodent line.