User blog comment:Zathronas/Analysis by an author: volume 2 trailer part 2/@comment-10390252-20140708060243

Your theory about Neptune would hold more water if there was a common naming theme across the bad guys (narrowly, just Cinder's group) but that isn't the case. I suspect that it is more the characters having common stylistic themes than anything else:

Cinder - Cinderella

Roman - Candlewick from Pinocchio

My guess is that Mercury and Emerald will both turn out to have similar disney character themes as the overall basis of their design rather than necessarily the name.

FWIW, I think that Neptune is the fish guy from the Chinese story 'Journey to the West', which is also the most famous appearance of Sun Wukong, the King of the Monkeys.