Invisighouls are the minions of Hector the Reflector, and appear only on Hector's boss battle. Yoshi's Island DS, they don't attack Yellow Yoshi directly but they are only seen in the mirror on the lower screen. They also carry chairs, tables and flowerpots which can hurt Yellow Yoshi if he touches it however they can be swallowed along with the Invisighoul that is carrying it.
It's believed that Invisighouls are responsible for the cleanliness of the house. It is not known if Hector the Reflector treats them with respect and pays them above minimum wage.