Metaboss is an anthropomorphic pig in Mario & Luigi RPG 3!!! He is a minion of Fawful who roboticized and guarded Bowser's Castle, as well as fighting the Koopa King four times. When Bowser meets Fawful, he sends Metaboss to battle him. Fawful teaches Bowser how to use his action commands on Metaboss. Soon Bowser gets weakened by Metaboss.
Later in the game, he made Bowser's Castle animate. He lands the castle on top of Bowser. Bowser then becomes giant thanks to Mario and Luigi. Metaboss fights Giant Bowser with the castle. After being defeated, he retreats with the castle into a new area.
When Bowser reaches the castle, he realizes it's been completely changed by Fawful. Bowser finds out that he made a piece of Bowser's castle into a gigantic auditorium with a big stage at the end of it. Fawful is seen hosting some kind of show or act. When Bowser is about to fight Fawful, he sends Metaboss to battle him again in a cage match. In this battle, Metaboss attacks in various ways. He can attack by trying to body slam Bowser or tackling into him. He can also punch and do other various attacks. When he attacks Bowser perfectly, food and coins will be thrown to him. Metaboss can then eat the food to recover HP. Bowser can do the same thing and earn treats by doing a good move on Metaboss. After Bowser finally defeats Metaboss, he is rewarded with food. He then gets overstuffed and gets stuck in the floor.
Later in the game, Fawful and Metaboss see Bowser stuck. Metaboss gets him out and Bowser lands onto a treadmill making himself normal-sized again.
In Princess Peach's Castle, Metaboss made a trash monster to attack the Mario Bros. so they can't pass to rescue Bowser who is knocked out. After the defeat, he goes into another area of Princess Peach's Castle.
When Bowser meets with Metaboss again, Fawful powers up Metaboss making him completely blue, wearing a crown, and makes him a lot stronger than before. Bowser has to battle Metaboss in this form. Metaboss can now use ice attacks thanks to the power-up. He can jump onto a gigantic snowball and try to roll over Bowser with it. There is a Bob-Omb stuck in it. Bowser has to punch the Bob-Omb to make the snowball explode and make Metaboss lose HP. Metaboss can also send Fawful Snowmen out of his crown to attack Bowser. They can also throw little snowballs at Metaboss to make him recover HP. Metaboss also attacks by using ice powers and once again, body slamming into him. Bowser's Fire Breath can cause a lot of damage to Metaboss. After Metaboss is defeated once again, he gets frozen in a gigantic block of ice. There is also bits of ice being blown out of Metaboss' Snout. Bowser needs to use the ice to make the Mario Brothers progress inside of Bowser's body.