A Foo is an enemy in the Mario (Series), that first appeared in the game New Super Mario Bros. Wii, and that are very similar to Fwooshs. They are cloud like enemy that attacks Mario by shooting out fog so that Mario can't see. Mario can clear the fog by doing a Spin.
A Muckroom Kingdumb version of the Foo exists; it is called the Fwuh.
There is also a special Foo called Fooloo in Super Mario Starblast. They have mouths on both sides and blows fog from both sides.