From the Super Mario Wiki
Bowser's Claw is a large mechanical hand that made its debut in New Super Mario Bros. U and New Super Luigi U. As the name implies, the hand is modeled after Bowser's actual claws. It seems to be an attachment to Bowser's Flagship, and it is most likely operated by Bowser Jr.. It is present during the intro, where it tosses Mario, Luigi, Yellow Toad, and Blue Toad, into the Acorn Tree, The Mighty Cannonship (All Aboard! in New Super Luigi U), where it attempts to knock the players out of the airship, and Boarding the Airship (Bowser Jr. Showdown in New Super Luigi U) where it acts similarly to the Smashers in Super Mario World, and the Chomps from Super Mario World 2: Yoshi's Island. Ultimately, the hand destroyed the Flagship when it attempted to crush the players while they were on board.