Pikachu's Ocean Adventure
PK16 doesn’t quite fit with it’s release date due to the inclusion of
Ash’s Aipom. The mischevious pokémon first appears one episode later,
but is not captured until AG182.
The trailer appears to show Aipom fully integrated with the group, so it seems likely that this takes place after her capture. It’s worth noting that other lists prefer to place this short immediately after Aipom’s debut, however due to the trailer evidence it will be placed post-capture.
Source: YouTube