From the Super Mario Wiki
A Little Skull Mouser is an enemy found only in Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3 that is similar to Little Mouser. It jumps aimlessly and, like most enemies in the game, can knock Baby Mario off a Yoshi's back. Unlike regular Little Mousers, Little Skull Mousers are unable to steal Yoshi Eggs.
When a Yoshi jumps on a Little Skull Mouser's head, hits it with an Egg, or spits an enemy at it, the skull it is wearing crumbles, turning the creature into a normal Little Mouser. Unlike many other small enemies, a Little Skull Mouser cannot be defeated immediately by a Ground Pound; instead, the attack has the same effect on it as a normal jump, merely destroying the skull on its head.
Names in other languages
|From Hone (Bone) and Chū-san (Little Mouser's Japanese name)
||From "Bad Rat", the German name for Little Mouser