Mecha-Bowser

Mecha Bowser is a huge Bowser-shaped robot first used by Bowser Jr. to attack Mario and escape from him in Super Mario Sunshine, but it has since made several appearances in some form or another in other games, including another main series title and various spin-offs.

Super Mario Sunshine
Mecha Bowser is found in the first episode of Pinna Park and is piloted by Bowser Jr. To fight it, Mario must board a rollercoaster and fire Water Rockets (via F.L.U.D.D.) at Mecha Bowser when he gets close enough. Mecha Bowser will attack by shooting Bullet Bills at Mario, and by breathing fire on the rollercoaster tracks. After enough hits, the mech will explode, and Bowser Jr. will use its head to escape.

Mario Power Tennis
In this game, Mecha-Bowser is the boss of the Mecha-Bowser Mayhem minigame, in which it tilts 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 its rampage is quelled.

Mario Party series
In Mario Party 5, Mecha-Bowser is a giant toy that shoots characters out of a cannon. 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 whenever players land on Jackpot+ Spaces, and players can earn these stars by playing minigames or by landing on a Jackpot Space.

Super Mario Galaxy
Mecha-Bowser appears in the Toy Time Galaxy; however, its appearance has been drastically altered from that in previous games. It is now a toy robot and, out of all of Mecha-Bowser's incarnations, resembles Bowser the least. It also sports a blue and green color scheme. To "defeat" Mecha Bowser, Mario must climb on its head, dismantling portions on his way, and then Ground Pound through the rocks on top to cause the robot to explode and free the Gearmos inside it.

Trivia

 * Mecha Bowser is very similar to Mechagodzilla from the Godzilla series. Both are robotic versions of a giant reptile that has appeared earlier in their respective series and when Mecha Bowser appears, the theme is very reminiscent of the Mechagodzilla appears theme from Godzilla vs. Mechagodzilla.
 * Even though they don't take on this particular Mecha-Bowser in terms of appearance and aesthetic, Bowser???, King Boo's Bowser decoy, and the False Bowsers were the other previous instances of large mechanical Bowser lookalikes operated by other enemies (the latter two being magically operated to varying degrees), similarly to Mecha-Bowser, predating Super Mario Sunshine.