Mecha-Bowser

Mecha-Bowser, also known as Mechakoopa, is a huge Bowser-shaped robot that made its first appearance in Super Mario Sunshine. It has since made several appearances in some form or another in other games, mostly various spin-offs. It may be based on the kaiju monster.

Super Mario Sunshine
In Super Mario Sunshine, Mecha-Bowser is found in Episode 1 of Pinna Park and is controlled by Bowser Jr. To fight it, Mario, riding the park's Roller Coaster, must collect and fire water rockets (via FLUDD) at Mecha-Bowser. Mecha-Bowser attacks by shooting Bullet Bills at Mario, and by breathing fire on the Roller Coaster track, which can be doused using FLUDD. After enough hits, the robot explodes, and Bowser Jr. pilots its head away to escape.

Mario Golf series
While Mecha-Bowser does not physically appear in the series, its name appears on scoreboards in Mario Golf: Toadstool Tour and Mario Golf: Advance Tour.

Mario Power Tennis / New Play Control! Mario Power Tennis
In Mario Power Tennis and its Wii port, Mecha-Bowser is the boss of the Mecha-Bowser Mayhem minigame, in which it rotates the court and battles players, firing Bullet Bills and Bob-ombs that can be deflected back at it. Players must attack it with charged tennis balls until it is defeated.

Mario Party series
In Mario Party 5, Mecha-Bowser is a giant toy on the Toy Dream board and shoots characters out of a cannon in its mouth to another location on the board whenever they land on a nearby ? Space. In Mario Party 7, its head is a feature of Bowser's Enchanted Inferno!. In Mario Party 9, its head is featured on the Bowser Station board as a Jackpot Machine. It stores Mini Stars, and players can earn these stars by playing minigames or by landing on Jackpot+ Spaces.

Mario Kart Arcade GP series
In Mario Kart Arcade GP and Mario Kart Arcade GP 2, when finishing the Bowser Cup with Bowser, Mecha-Bowser replaces the giant Bowser in a boss fight. His tactics are the same as Bowser. The player has to throw Black Shells at the pillars supporting Mecha-Bowser to defeat him.

Other appearances
In the Super Mario Mash-up in Minecraft, Mecha-Bowser's head replaces the original game's furnace block.

Trivia

 * When firing Bullet Bills or damaged, the cannons shown resemble the controller ports on the Nintendo GameCube.