King Mackerel the Second appears in Buster's dreams in "Buster Baxter and the Letter from the Sea". He is a merman who rules the kingdom of Atlantis at the bottom of the sea.
Buster assumes that a message in a bottle, which was in fact sent by a young Captain Jack, is a message from Atlantis. He dreams that King Mackerel is asking for help, but does not know what he is supposed to do. He tries sending King Mackerel messages in bottles which end up littering the beach. In a second dream, King Mackerel tells Buster to clean up the beach, which he does with Arthur's help.