This Croagunk is Looker's only Pokémon in the main series games, first appearing in Platinum and battling Team Galactic to stop Heatran. It is his invaluable partner Pokémon, always being by his side.
Sometime between the events of Black & White 2 and X & Y, it is implied that Looker's Croagunk had been killed. Looker is shown to have been affected by this, as his personality had changed quite a bit.
Croagunk appears in Pokémon Generations, going through the same events as Platinum, and also is Looker's Sync Pair in Pokémon Masters.