I've seen many fanart works of Oshawott and Popplio together on DeivantArt. Typically, these involve Oshawott (or one of its evolutions) consoling Popplio by saying Oshawott was hated back in its day also.
I can imagine that much of Greninja's popularity is probably due to Ash-Greninja-it's gotten to the point where that particular form is showcased in the Sun/Moon demo.