Skipsqueak

Skipsqueaks are mouse-like enemies that appear in Super Mario 3D World. Their name is a portmanteau of "skip" and "pipsqueak".

Super Mario 3D World
Skipsqueaks constantly run on moving platforms going in the opposite direction, causing them to remain in one place. They also jump up periodically and are usually found in groups of two or more. Skipsqueaks can be defeated by using any form of attack, including a jump attack. They appear in the levels Really Rolling Hills, The Bowser Express, Grumblump Inferno, and Rainbow Run.

There is a variation of this enemy known as Spiny Skipsqueaks which appear in Cookie Cogworks and Rainbow Run.

Super Mario Maker 2
Skipsqueaks and Spiny Skipsqueaks reappear as enemies that can be placed in Super Mario Maker 2 stages. Skipsqueaks jump whenever Mario jumps, and run in place to keep up with Conveyor Belts. If a Skipsqueak is pushed away from its starting point, such as by a Thwomp or Track Block, it will run back towards its starting point and even Triple Jump over obstacles to get there. If it cannot pass an obstacle, it will give up and look tired, then reset its movement pattern from its new location.

Super Mario Maker 2

 * North American website bio: "Skipsqueaks like to try and match Mario’s jumps. Sometimes you can use that to your advantage!"