A messenger appears in Buster's dreams in "Buster Baxter and the Letter from the Sea". He is a sea creature with a fish's head and a frog's hand and feet.
Buster believes, that he received a message from Atlantis. In his dreams, the messenger informs King Mackerel II of Atlantis of their unsuccessful attempts to contact Buster. Eventually, King Mackerel visits Buster in person and tells him to clean up the beach.