This Key is the key to open the door to Peach in Bowser's Catle. It is unknown why it's known as that, but he named it This Key. When Bowser showed it to Mario and company, Baby Yoshi ate it, yet he spitted it out. After Mario, Luigi, and Baby Yoshi used it to save Peach, it never appeared again. Despite this, it appeared as a boss in Mario versus his Replacements.