Nabbit

The Mushroom Bandit is a character introduced in New Super Mario Bros. U. As the name implies, he is a thief, similar to Bandit, and the bag he is holding is presumably filled with Mushrooms, most likely Super Mushrooms and 1-Up Mushrooms.

The mask he wears resembles the mask Bowser Jr. wears, so he presumably has an affiliation with the Koopa Troop.