From the Super Mario Wiki
Retrieved from "http://www.mariowiki.com/index.php?title=Anvil&oldid=1848931"
Anvils are weapons similar to Thwomps appearing in the final pages in Yoshi's Story, where they are carried by the Black Shy Guys inside Bowser's Castle. If Yoshi walks near the anvil-carrying Shy Guys, they will drop the giant, spiked anvil and crush whatever is beneath it. They appear in the levels Lift Castle, Ghost Castle and Mecha Castle.