Madzilla is a fictional monster who stars in his own movie series, which are filmed in Hollywood.
In The Adventures of Super Mario Bros. 3 episode "Dadzilla", Kootie Pie and Big Mouth, upon arriving in Hollywood, thought that Madzilla was their real father, due to the way that Bowser treated Kootie Pie earlier on. The two Koopalings used their Magic Scepters to bring a mechanical Madzilla to life. Confused upon being brought to life, Madzilla was tricked by Big Mouth and Kootie Pie into thinking that they were his long lost children, and they told him what parents are "supposed" to do. At first, Madzilla was not evil, and only wished the best for his "children", such as stealing a car and towing them around Hollywood.
Eventually, Mario, Luigi, Princess Toadstool and Toad caught on to the two Koopaling's plans, and come to stop it. Using a Fire Flower, Mario became Fire Mario, but was unable to stop Madzilla before losing his powers. Madzilla managed to lock the heroes up in a props warehouse, and allowed Big Mouth and Kootie Pie to get their own clawprints in the Walk of Fame.
Mario and his friends then built their own robot out of spare parts in the warehouse to play the role of Madzilla's Mom. In the interum, Madzilla was told to steal the Hollywood Bowl so that Big Mouth and Kootie Pie could perform their own concert. After the concert received a poor reception from the public, Madzilla picked up several innocent people, whom he threatened to toss in the Pacific Ocean. Before he could do so, Madzilla met up with Mario's robot, which he believed was his mother. While controlling Madzilla's Mom, Mario tricked Madzilla's "children" into fleeing by insisting that everyone in their family must take naps so that they'll be rested to do their chores. Not wanting to stick around, Big Mouth and Kootie Pie ran back home.
Afterwords Mario, Luigi, Princess Toadstool, and Toad explained everything to Madzilla, and allowed him to live in Giant Land, to which he happily accepted, especially once he found out that the Koopalings would no longer bother him. It is presumed that he still lives in Giant Land.