What's a Milotic doin' in Gensokyo? I'm pretty sure it ain't forgotten. Unless it was previously used in competitive play and now it ain't?
Well, it dropped from OU in RSE to UU/OU in DPP/BW, and as of XY is in UU pretty much only. So it did drop in popularity, but nowhere near the level of being "forgotten". So maybe it's just a gratuitous appeatance.