BWAMPSHAT was a Pokémon distributed for April Fool's day. Given its temporary nature, it does not register in the Pokédex, to prevent the Pokédex from being permanently incomplete for players that missed the event.
BWAMPSHATs only appeared as a distinct Pokémon on April 1st of 2017. Any games that were updated to receive the data for BWAMPSHAT were also set to delete the data on April 2nd. To prevent any errors in the game, BWAMPSHATs were created as "alternate forms" of various other Pokémon (such as Swabone and Magikarp). As such, after their data was removed, they would appear only as a regular Pokémon.