From the Super Mario Wiki, the Mario encyclopedia
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Wereturtles are a type of bestial Koopa, having fur and retractable claws; Wereturtles, instead of being a distinct type of Koopa, are simply a form taken by Koopa Troopas after they stare at the full moon.
In The Super Mario Bros. Super Show! episode "Count Koopula", Count Koopula, wanting to capture Princess Toadstool, has several of his Koopa Troopa henchmen change into Wereturtles by looking at the full moon, and proceeds to send them out to kill Mario, Luigi, Toad and bring him Princess Toadstool.
Charging into Count Koopula's castle, these Wereturtles go directly to Count Koopula's tomato sauce cellar, where they subsequently slip on some tomato sauce spilled on the floor of the cellar by Mario, Luigi, Toad and Princess Toadstool; after slipping, these Wereturtles crash headfirst into some nearby shelves containing bottles of tomato sauce.
One of these Wereturtles manages to recover from this crash though, and tries to attack Toad, Mario, Luigi and Princess Toadstool; this Wereturtle is defeated by Toad, who throws a bottle in the Wereturtle's open mouth, which it swallows and begins to choke on.
Later, when Count Koopula corners Mario, Luigi, Toad and Princess Toadstool in his basement, he has several Wereturtles, as well as zombified Goombas, in his company.