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're 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.
It is possible that the chairs and flowerpots that seem to move on their own in various levels could be moved by these; however, this cannot be proven for certain, as there are no other mirrors in the game.