So as those of you who have been Gen VI battling might have noticed, Greninja is able to learn Rapid Spin on Showdown. However, serebii has put up the learnset of Greninja and it doesn't have Rapid Spin as one of its moves.
Um... I tried to put rapid spin on greninja, it said it was illegal, and it wouldn't let me battle either, so I don't think it can learn rapid spin
Source: My trial on showdown