Mecha-Bowser

Mecha Bowser is a huge Bowser-shaped robot used by Bowser Jr.. Mecha Bowser is vaguely similar to Giga Bowser, because of height, weight, and appearance.

Super Mario Sunshine
In Super Mario Sunshine, Mecha Bowser is found in the first episode of Pinna Park, being controlled by Bowser Jr. To fight him, 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. Eventually, Mecha Bowser is destroyed by Mario.

Super Mario Galaxy
In Super Mario Galaxy, Mecha-Bowser appears in the Toy Time Galaxy; however, he appears very different from how he has appeared in the past games. In this game, instead of a robot, it is a toy robot and it has little resemblance to Bowser compared to the previous versions. This is probably because this Mecha Bowser is blue and green instead of the normal Bowser colors. To "defeat" Mecha Bowser, Mario must climb to the head of Mecha Bowser (similar to Megaleg) and then Ground Pound through the rocks on top and into its head to destroy it and free the Gearmos that are captured inside it.

Mario Power Tennis
In Mario Power Tennis Mecha Bowser appears as a Boss in a minigame and appears in background of a certain court.

New Play Control:Mario Power Tennis
In New Play Control:Mario Power Tennis he once again appears as a boss minigame

Mario party Series
Mecha Bowser Has appeared in 3 Mario Party games.In Mario Party 5 he is an NPC in Toy Dream,in Mario Party 7 it's head cameos in Bowser's Enchanted Inferno!,and more recently in Mario Party 9 Its head is in the stage Bowser Station although slightly altered into "A Jackpot Machine".

Mario Golf Series
In Mario Golf: Advance Tour and in Mario Golf: Toadstool Tour its name appears on score boards.

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.