An impostor disguised as Bowser who guards the castle at the end of Worlds 1 through 7. Each fake Bowser is actually a regular enemy — a Goomba, Koopa Troopa, Buzzy Beetle, or similar — wearing a Bowser costume. They breathe fire and move like Bowser but are revealed to be impostors when defeated with fireballs.
Reach the axe at the end of the bridge to drop the fake Bowser into the lava — this works on all of them. Alternatively, 5 fireballs will reveal their true form. Each castle's fake Bowser moves slightly faster than the last.