Midbus

Midbus (Metaboss in Japan) is an anthropomorphic pig in Mario & Luigi: Bowser's Inside Story. He is a minion of Fawful who robotized and guarded Bowser's Castle, as well as fighting Bowser three times. When Bowser meets Fawful, he sends Midbus to battle him. Fawful teaches Bowser how to use his action commands on Midbus. Soon Bowser gets weakened by Midbus.

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. Midbus 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 Midbus to battle him again in a cage match. In this battle, Midbus 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. Midbus can then eat the food to recover HP. Bowser can do the same thing and earn treats by doing a good move on Midbus. After Bowser finally defeats Midbus, he is rewarded with food. He then gets overstuffed and gets stuck in the floor.

Later in the game, Fawful and Midbus see Bowser stuck. Midbus gets him out and Bowser lands onto a treadmill making himself normal-sized again.

In Princess Peach's Castle, Midbus 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.

Blizzard Midbus
When Bowser meets with Midbus again, Fawful powers up Midbus, changing him into Blizzard Midbus. In this form, Midbus is completely blue, with a brown back, wearing a crown, and is a lot stronger than before. Bowser has to battle Midbus in this form, which is difficult since Midbus 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 Midbus lose HP. Midbus can also send Fawful Snowmen out of his crown to attack Bowser. They can also throw little snowballs at Midbus to make him recover HP. Midbus also attacks by using ice powers and once again, body slamming into him. Bowser's Fire Breath can cause a lot of damage to Midbus. After Midbus is defeated once again, he gets frozen in a gigantic block of ice. There is also bits of ice being blown out of Midbus's Snout. Bowser needs to use the ice to make the Mario Brothers progress inside of Bowser's body.

Trivia

 * Even though it may just be a coincidence, the sign on Midbus's arm looks similar to the logo seen on Pigmask machinery in Mother 3.