Foo

Foos are white cloud-like enemies that first appeared in New Super Mario Bros. Wii. They produce fog that can obscure the player's view. Foos can be defeated with any kind of attack. Their name comes from Fwoosh, a similar enemy from Super Mario 64 and its remake that blows wind.

New Super Mario Bros. Wii
New Super Mario Bros. Wii marks Foos' debut appearance. They appear only in World 7-5, and create fog periodically. The fog vanishes after a few seconds and can be blown away by Spin Jumping, or by propelling as Propeller Mario.

New Super Mario Bros. U
In New Super Mario Bros. U, Foos appear only in Snaking above Mist Valley and behave the same as they did previously.

New Super Luigi U
Foos also appear in New Super Luigi U. Here, they are only found in Three-Headed Snake Block.

Mario Party 10
Foos play a role in the Mario Party 10 minigame Foo Me Once, where they blow fog to obscure the buttons displayed on the platforms which the players need to remember in order to safely traverse across.

Dr. Mario World
In Dr. Mario World, Foos appear in the background of World 15 once an area has been cleared.

Trivia

 * In some early trailer footage of Super Mario Maker 2, the white part of Twister had a cloudy appearance resembling a Foo with elements of Ty-foo's design. This resemblance was closer in the New Super Mario Bros. U style, where they lacked the blue cheeks and had pink lips. This design seems to have been entirely overwritten, as it is not present in the final game's data.