From the Super Mario Wiki
|The Invisighouls are invisible ghosts. They only appear during the boss battle with Hector the Reflector in Yoshi's Island DS. They look like small versions of Hector, but are only visible in the Mirror. They sometimes throw chairs and flower pots at Yoshi during the battle. However, they are more of a help than a hindrance, as Yoshi can easily gobble up the furniture they throw at him and convert it into ammo that he can use to defeat Hector.
The three types of Invisighouls