If you mean this one
After looking at the statue in the Poké Mart, at the Pastoria Great Marsh in Platinum, if the player stands behind the Croagunk Wall with the player's head sticking out of it, a wild Croagunk will come, but it will just cry and walk into the Great Marsh building.
I think its Looker or Looker's Croagunk. When I went to Stark Mountain, Looker takes the Magma Stone from Charon. I have tried looking through the board in Pastoria after beating Stark Mountain, but the Croagunk doesn't pass by again.