World 3 (New Super Mario Bros.)

Somehow Mario and Luigi ended up in an island in World 3 of New Super Mario Bros.. The island had Bowser Jr. and a Ghost House! The boss of this island was Cheepskipper the giant Cheep Cheep.

Mario and Luigi can beat the game without needing to beat this world. If Mini Mario or Mini Luigi defeats Mummipokey in World 2 he will go to World 4 instead.