Little Skull Mouser
From the Super Mario Wiki, the Mario encyclopedia
A Little Skull Mouser, also known as a Little Skull Monster, is an enemy found only in Super Mario World 2: Yoshi's Island, Yoshi's Island: Super Mario Advance 3, and Yoshi's New Island 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 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.
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