Pokestar studio Pokemon are used in some randomized formats.
Glitch Pokemon like missing no are used in some random formats.
Cosplay Pikachu can be used in any Gen 6 format it is not banned in.
Spiky Eared Pichu can be used in any Gen 4 format it is not banned in.
Eternal Floette (not Eternal Flabébé) can appear in some random formats.
(For example: Randomized formats like Metronome and Hackmons Cup, which can use all Pokemon except CAP)
Source (I looked through there), Experience. :P
Hope I helped!