From the Super Mario Wiki
Cheepskipper is a large purple and white Cheep-Cheep with yellow fins and two sharp fangs protruding from its bottom lip. Its sole appearance to date is as the boss of World 3 in New Super Mario Bros.
Cheepskipper jumps out of the water on the right side of the screen, bounces along the bridge and exits from the left. While it is out of the water, Mario or Luigi must jump on it three times. Cheep-Cheeps also jump out of the water during the battle, and cause damage if they hit Mario or Luigi.
Defeating Cheepskipper opens the path to World 5.
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