Wendy Koopa is the third boss of the game, and the first to pose a real challenge, in my opinion.
You have to worry about the bouncing rings, which can mess you up when you're trying to focus on Wendy. The trick is to take her down quickly before the boss battle is filled with those blasted rings. A good and challenging fight for the NES Mario days.
You then save your third king...wait a minute. He looks like Mario!