Verified answer

Croagunk is one of the random extra Pokemon in the Safari Zone that changes each day. To see what Pokemon is there, check the binoculars at the top floor.

I don't think you can find one in the safari zone area. There are Croagunks in Pastoria's Great Marsh, on Iron Island, On Mt. Coronet and in the Lake Valor area.

Have you tried going up to the observatory and having a look around to see where the best place is to catch him?

Are you planning to just see it or capture one?

If you plan on seeing one, just battle trainers. Try areas near wet places...