Baby Bowser is Bowser's baby self who first appeared in Super Mario World 2: Yoshi's Island. With the help of Kamek, he captures Baby Luigi and the Stork. Although Kamek does most of the work to stop Baby Mario and Yoshi, Baby Bowser fights Baby Mario in the final boss battle. He also appears in Yoshi's Story.
NOTE: Baby Bowser is NOT Bowser Jr., who is Bowser's son; nor is he a Koopa Kid, who is a miniature version of Bowser.