Fake Bowser

A fake Bowser (also called Bowser Impostor, Bowser clone or Bowser's decoy) is a standard enemy that was transformed by Bowser's black magic into a copy of himself. They first appear in Super Mario Bros.

Concept and creation
The concept of fake Bowsers came about during development of Super Mario Bros.. Since all enemies have their sprites flipped upside-down when they are defeated (except for Piranha Plants, which simply disappear, and Bullet Bills, which stay rightside up), one of the programmers accidentally inserted an upside-down Koopa Troopa shell when portraying Bowser's death animation as if he were defeated with fireballs. The developers liked this idea, and as a result, this idea was extended to most of the Bowser battles in the game.

Super Mario Bros. / Super Mario Bros.: The Lost Levels
In Super Mario Bros., the seven fake Bowsers are placed in each castle, with Bowser's Castle containing the real one. Defeating a fake Bowser with five fireballs reveals that they are really just common enemies disguised as Bowser. The enemies and their abilities are the following: Super Mario Bros.: The Lost Levels additional worlds (in the Super Mario All-Stars version, the final fake Bowser is now replaced with the actual Bowser): The "You VS. Boo" challenge of Super Mario Bros. Deluxe: Like Bowser, the blue Bowser's Brother in World D-4 is also a fake revealed to be a Spiny when defeated with fireballs, and only breathes fire. In Super Mario All-Stars, both of these fakes are replaced with the real ones that only throw hammers, although Bowser's Brother is now green and looks identical to his twin. Due to All Night Nippon: Super Mario Bros. also featuring Worlds A-D, all of the fake Bowsers from those worlds return, including the fake Bowser's Brother in World D-4.
 * World 1: Little Goomba - fire
 * World 2: Green Koopa Troopa - fire
 * World 3: Buzzy Beetle - fire
 * World 4: Spiny - fire
 * World 5: Lakitu - fire
 * World 6: Bloober - hammer
 * World 7: Hammer Brother - hammer
 * World A: Goomba - fire (FDS) / Red Koopa Troopa - hammer (All-Stars)
 * World B: Green Koopa Troopa - fire (FDS) / Green Cheep-Cheep - hammer (All-Stars)
 * World C: Buzzy Beetle - fire (FDS) / Bullet Bill - hammer (All-Stars)
 * World D: Spiny - fire (FDS)
 * 1-4: Green Cheep-Cheep
 * 2-4: Bullet Bill

Super Mario Advance 4: Super Mario Bros. 3
In Super Mario Advance 4: Super Mario Bros. 3, a fake Bowser appears as a boss in the World-e level Classic World 1-4, where it plays a similar role to that played in Super Mario Bros., though it looks identical to the game's real Bowser. It is revealed that the fake Bowser is a Goomba if it is defeated with fireballs.

Super Mario 3D Land
While typical fake Bowsers do not appear in Super Mario 3D Land, variants powered by Super Leaves known as Tail Bowsers make their first appearance.

Super Mario Run
In Super Mario Run, fake Bowsers appear at the end of the fortress levels in Worlds 1, 3 and 5. They are a Goomba, a green Koopa Troopa, and a Buzzy Beetle, respectively.

Trivia

 * The World 3 and World C (original) fake Bowsers are the only Buzzy Beetles that can be defeated by fireballs (outside of Super Mario Bros.: Peach-hime Kyūshutsu Dai Sakusen!), and the World C (All-Stars) and "You VS. Boo" 2-4 (Deluxe) ones are the only Bullet Bills that can be defeated by fireballs.
 * The World A and World C fake Bowsers in All-Stars are the only fake Bowsers whose true forms are right-side-up instead of upside-down.
 * In Mario & Luigi: Superstar Saga and Mario & Luigi: Superstar Saga + Bowser's Minions, an exaggerated cardboard Bowser suspended by a pair of ropes over a bridge is seen at the end of a special area inside Bowser's Castle, which is a side-scrolling level imitating a Super Mario Bros. castle. Mario must move underneath the stationary object and touch the axe to make it fall.
 * If the player beats Bowser's Sky Scuffle with any character barring Bowser Jr. in Mario Party: Island Tour, Bowser will reveal that the "Bowser" they defeated had been a decoy, which may have been a reference to the fake Bowsers.