From the Super Mario Wiki
Little Mousers are a species of small, blue or purple mice-like creatures who wear bandanas that obscure their faces. Their English name refers to another mouse-like foe, Mouser, who was introduced in Super Mario Bros. 2.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Little Mousers first appear in Super Mario World 2: Yoshi's Island, where they attempt to steal Yoshi's eggs and scurry away with them into their burrows. Ordinarily, they appear blue with yellow ears and orange feet. However, in a secret area of KEEP MOVING!!!! they appear gray with teal ears and turquoise feet. Like many blue enemies in the game, their artwork instead depicts them as purple. Little Skull Mousers also appear, and are just like normal Little Mousers, except they wear skulls on their heads, move solely by jumping, and will not steal eggs until the skull is removed from the Little Mouser.
Yoshi's New Island
Little Mousers, along with Little Skull Mousers, both under their original name, also appear in Yoshi's New Island. They act the same as in their first appearance.
Yoshi's Woolly World / Poochy & Yoshi's Woolly World
Yoshi's Crafted World
Mario Kart: Super Circuit
Little Mousers later appear under the name Mice in Mario Kart: Super Circuit as obstacles in the Cheese Land course. However, they are replaced by two Chain Chomps in the course's reappearance in the Animal Crossing × Mario Kart 8 DLC pack for unknown reasons.
Paper Mario series
Little Mousers and a new sub-species known as the Nomadimouse appear in Paper Mario as friendly NPCs, where they are found in Dry Dry Outpost. One of them named Moustafa gives Mario the Pulse Stone, which is necessary to open the secret path to Dry Dry Ruins.
Paper Mario: The Thousand-Year Door
In Paper Mario: The Thousand-Year Door, Little Mousers were renamed Squeeks and were shown to be one of the primary inhabitants of Rogueport and Rogueport Sewers; a white Squeek, known as Ms. Mowz also appears in Paper Mario: The Thousand-Year Door as an optional partner of Mario.
Mario Party series
Mario Party Advance
In Mario Party Advance, a Little Mouser named Mouser appears as the boss and owner of the Duel Tower. He challenges the player to the game Chicken!. If the player wins, he receives the minigame. The ending said that he was bummed that Whomp left the Duel Tower.
Mario Party 8
In Mario Party 8, Little Mouser (called Mowz) is a recurring character. It first appears in King Boo's Haunted Hideaway. If a ? Space is landed on inside the mansion, Mowz will let the player spin a wheel with a pinball inside. Depending on which one of three holes on the wheel the ball lands in (Coins, Star, or Candy), it will steal an item from a random opponent for the player.
Another appearance of Mowz is in Shy Guy's Perplex Express. It stole Holly Koopa's candy, and when landing on a ? Space above the train, a Shy Guy will come and tell the character about the incident. Then he'll tell the character who the suspect is (Bandit can also be the one who stole the candy). If the player correctly remembers which car Mowz was on, they will be rewarded with 20 coins (10 in Duel Mode).
Super Smash Bros. Ultimate
Profiles and statistics
Super Smash Bros. Ultimate spirit
Yoshi's Crafted World
Names in other languages