Peacekeeper

The Peacekeeper is a robot powered by the mysterious "Red Core Energy" built by President Stone. It also has "Prototype Adaptive Technology" that allows it to take in other things and objects. Originally built to help Stone get re-elected, it began destroying Metro City due to the Red Core's influence and Astro was forced to destroy it

Movie
When the Peacekeeper was complete, the science institution was going to place the Blue Core into it which contained positive energy. However, Stone, who wanted it to be a war machine, put the Red Core in it. The Peacekeeper then went on a rampage as a result and the scientists locked it in a force field and knocked it out when it tried to use its firepower to get out of the institution.

Later, when Astro fled from Stone, the mad president activated the Peacekeeper with the red core again, despite having seen the previous results. The Peacekeeper paid President Stone back by absorbing him and upgrading itself with other parts of the area around it. The Peacekeeper, after accidentally scaring off an entire press conference, roared at some attacking ships and absorbed them to make himself stronger. The eventual result is an amalgamation of the machines (along with some parts of the city) turned into a colossal mechanical monster with Stone's mind controlling it. It then went to the surface world to get Astro. Peacekeeper-Stone wasn't able to absorb Astro, because he had the Blue Core and both combatants could be destroyed if their Cores came in contact with one another. Willing to give his life to save the inhabitants of Metro City, Astro plunged into the robot and destroyed it by overloading its systems with the Blue Core. Astro and President Stone both survived the ordeal with the latter being arrested for the destruction he had caused.

Video Game
The Peacekeeper appears as a recurring boss in the game, as the boss of World 4, a mini-boss in World 5 as a slightly larger version of himself, and the final boss of the game as a horrendous gargantuan mechanical monster.

Trivia

 * Hilariously, after combining with random metallic parts across Metro City, the Peacekeeper has a toilet on its back
 * The Peacekeeper bears a resemblance to the Iron Giant with its arm cannon
 * Its absorbing ability is similar to that of Devastator from Transformers: Revenge of the Fallen