Other Pokemon to collect include the 28 different variants of Unown, the ones with gender variants, regional variants, Wormadam and Rotom variants, and the 20 different variants of Vivillon. Although Castform have a few different forms, this is only temporary and it will revert to its original form after the event which triggered the change.
Then there are the extra Legendaries which are caught or traded for from the different Pokemon games (not all have been kept here, as some are still kept in game).
And then there are Shiny Pokemon which differ from the normal variant of the Pokemon that is normally caught.
As I have mentioned before, Pokemon is a great past-time for those who enjoy collecting.