Just like in "The House of the Lucky Gander," Launchpad has his own adventure which none of the other characters know anything about.
While many people believe that Cousin Fethry is a new character made up for the show, this is not true. In reality, going back to the 1950s Ducktales comics and going around the world, Cousin Fethry is well-known and popular. In fact, in Brazil, he had his own comic book series.
Huey is right about the dewfish being another name for a fresh-water catfish.
Fethry, the deep sea explorer, is voiced by experienced voice actor Tom Kenny, whose most famous role is as the titular character of Nickelodeon's SpongeBob SquarePants (1999), who lives in the sea. When we first see Fethry he's seen doing testing upon some sponges, possibly paying tribute to Kenny's character.
Fethry Duck's first appearance in Animation.