Invisighouls are the minions of Hetor the Reflector, and appear only on Hector's boss battle. They also appear in levels of Yoshi's Island DS, which are 2-3 and 5-4. In 5-4, they don't attack Yoshi that much as the previous levels. You cannot see them in Yoshi's Island DS; the items just float.
However they can poop on Yoshi.