From the Super Mario Wiki
Snooze-A-Koopas (also known as Snooza Koopas) are bizarre-looking Koopas encountered by Mario that only appear in Super Mario Sunshine and Super Mario 3D World. Snooze-A-Koopas, unlike their relatives, have flippers instead of hands or feet, and walk on all fours, similar to actual sea turtles. They are also quite large and have a shell with a Yoshi Egg-like pattern on it. True to their name, Snooze-A-Koopas are mostly found asleep; Mario can rouse them with a blast from F.L.U.D.D. When awake, they will attempt to jump on Mario. If they miss and land in the sand, they will become vulnerable to a Ground Pound.
Mario about to do a Ground Pound on a flipped Snooze-A-Koopa.
Mario's first and only encounter with Snooze-A-Koopas is in the fourth episode of Pinna Park, where he must save several Sunflower Kids who are wilting due to a large gang of Snooze-A-Koopas nibbling on their roots. After finishing this episode, Yoshi will be unlocked. Yoshis weren't available previously because they fled Isle Delfino when Snooze-A-Koopas invaded their habitat, partly because Snooze-A-Koopa's shells resemble Yoshi Eggs. In fact, a Toad tells Mario that Yoshis used to live on the island. They once again inhabit the island once Mario gets rid of the Snooze-A-Koopas, making it possible that they are predators of the Yoshis. They are a little similar to Electro-Koopas.