Croagunk is a pokemon that can be encountered after beating Corey's gym. It is the Croagunk that he uses in battle. When fought it is level 28 and does not carry any items. To encounter it you need to go past the beryl bridge and It is required to have rock smash. It is also required to have the silver ring which is found in cory's room after beating him. Go through the cave and then you will see Croagunk.