False Bowser is a boss found in Super Mario Bros. and Super Mario 3D Land. As the name suggests, False Bowser is one of the real Bowser's minions disguised as Bowser with the help of his magic, or in the case of Super Mario 3D Land, a Super Leaf.
Super Mario Bros.
False Bowser appears in Worlds 1-7 as the boss of the castle. With each world, the battles become increasingly difficult as Bowser begins to breathe more fire and throw hammers. To defeat him, Mario must wait for False Bowser to jump so he can run underneath him (or the player can use a well-timed jump to go over False Bowser). After jumping over False Bowser, Mario must pull away an axe holding the floor intact, causing False Bowser to fall into the lava.
Mario can also defeat False Bowser with the Fire Flower. If he uses this tactic, False Bowser will eventually stop attacking, and explode, revealing a Goomba that falls off-screen, revealing False Bowser's true form.
Super Mario 3D Land
False Bowser appears the castle boss of the game. False Bowser shoots fireballs at Mario, and jumps around to land on top Mario or block him from getting through. At close range, Bowser will also sweep Mario with his tail to damage him. Unlike Super Mario Bros., False Bowser is distinguishable by the raccoon tail where his regular one should be. Mario must run and jump across platforms (and later pillars) to get to a switch that will destroy the ledge Bowser stands on, causing him to fall into the lava. After falling into the lava, False Bowser will leap out of the lava and turn back into a Goomba and a Super Leaf, both of which fall into the lava and are destroyed.
In this game, False Bowser is immune to attacks from fireballs, boomerangs, tanooki tails, and statues.