Axe
The Ax is an item in the castle levels of the original Super Mario Bros. Mario or Luigi could defeat Bowser or his duplicates by touching the Ax, which would cut the support of the bridge Bowser was standing on and send him falling into the lava below. When Mario and Luigi did not have the Fire Flower, using the Ax was the only way they could defeat Bowser.
Later on, an Ax would appear as Treasure J in Course 29 of Super Mario Land 3: Wario Land. Another Ax played a similar role in Wario Land 3, where it served as Out of the Woods's Gray Treasure. With this Ax, Wario cut down a tree blocking him from The Peaceful Village and The Vast Plain.
Due to the nostalgia of Super Mario Bros., the Ax (which played a major role in its boss battles) has managed to earn Bowser-related appearances to this day. For instance, in Bowser's court in Mario Hoops 3-on-3, the basketball hoop's pole is a giant Ax. It also appeared in Mario & Luigi: Superstar Saga, where it cameoed alongside a paper duplicate of Bowser in a Super Mario Bros.-reminiscent section of Bowser's Castle. In New Super Mario Bros., the Ax was replaced by a Skull Switch in that game's Bowser battles.