Jelly Belly is a jellyfish.
"Jelly-Belly" is a large jellyfish with a fondness for peppermint. It was first encountered by Scrooge and co. on a mission to retrieve the Candy-Striped Ruby. It attacked Scrooge's submarine, but he gave it peppermint, causing it to release them. Later, the jellyfish, still searching for peppermint, attacked the ship of Porkman de Lardo and the Beagle Boys, and would not release them until Scrooge gave it more peppermint, which he did in exchange for the return of the Candy-Striped Ruby from De Lardo and the Beagles. The jellyfish was encountered once again when it saved Webbigail Vanderquack from falling overboard one of Scrooge McDuck's ships. It was nicknamed "Jelly Belly" by Webby, who fed it more peppermint in thanks.