Little Mousers are a mouse-like enemy that used to be part of the Mushroom Kingdom, but then abandoned it. They appear to be thieves that reside in Ghost castles in New Super Mario Bros. Wii. They are a sub-species of Mouser.
In the track Cheese Land, Little Mousers are obstacles that racers try to avoid. Running over one will make you spin out and lose Coins. They are there probably because Little Mousers like cheese. It would only make sense.
Little Mousers appears as friendly characters in this video game series. In Paper Mario, Little Mousers live all around the Mushroom Kingdom mainly at Dry Dry Outpost. Moustafa is a well-known Little Mouser that appears in the game.
In Paper Mario: The Thousand-Year Door, Little Mousers live in Rogueport. One Little Mouser even runs a Badge Shop near the Inn of Rogueport. Ms. Mowz is one notable Little Mouser who is a thief and is seen throughout the game. She eventually becomes Mario's partner after he does a trouble for her.