From the Super Mario Wiki
Wereturtles are a type of bestial Koopa, who, oddly, have more mammalian traits then reptilian ones, 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, had several of his Koopa Troopa henchmen change into Wereturtles by looking at the full moon and proceeded to send them out to kill Mario, Luigi, Toad and bring him Princess Toadstool.
Charging into Count Koopula's castle, these Wereturtles went directly to Count Koopula's tomato sauce cellar, where they subsequently slipped on some tomato sauce spilled on the floor of the cellar by Mario, Luigi, Toad and Princess Toadstool; after slipping, these Wereturtles crashed headfirst into some nearby shelves containing bottles of tomato sauce.
One of these Wereturtles managed to recover from this crash though, and tried to attack Toad, Mario, Luigi and Princess Toadstool; this Wereturtle was defeated by Toad, who threw a bottle in the Wereturtle's open mouth, which it swallowed and began to choke on.
Later, when Count Koopula cornered Mario, Luigi, Toad and Princess Toadstool in his basement, he had several Wereturtles, as well as zombified Gooombas, in his company.