Train Bandit

A Train Bandit is a chalk Shy-Guy etched onto certain walls in the game Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3. When Yoshi has transformed into the Train, Train Bandits come to life and chase after him. They are limited to the walls' borders and stop moving when Yoshi returns to his normal state. The only ways to defeat a Train Bandit are by using the Winged Cloud Maker, Anywhere POW, or by smashing a POW Block when they are on-screen, with the latter only being possible in the Game Boy Advance version, in Go! Go! Morphing!.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: These pistol-packing varmints try to hold up Yoshi after he's morphed into a steam engine.