Circling Boo Buddies
From the Super Mario Wiki
Circling Boo Buddies are a circular formation of Boos with different facial expressions first found in Super Mario World. They appear in small rooms in Ghost Houses and other haunted areas, including swamps and shipwrecks. Touching any one of the buddies can damage the Mario Bros. Unlike regular Boos, they do not become shy when looked at, suggesting Boos have more confidence in groups.
Circling Boo Buddies also make an appearance in World 5- of New Super Mario Bros. Wii, and in various haunted areas of New Super Mario Bros. U, where they can be found with different numbers of Boos and form larger or smaller circles.
Circling Boo Buddies reappear in Super Mario Maker, simply referred to as "Boo Buddies". They are variants of Boos, and can be made in the Course Maker by shaking a Boo. Like many objects, they can be enlarged with a Super Mushroom and given wings, which the latter will make them move in and out when they are circling. They can be placed in Warp Pipes, Bill Blasters, Blocks, Lakitu's Clouds and tracks. Because of the fact that Circling Boo Buddies (along with normal Boos) did not appear before Super Mario World, new sprites have been made for the Super Mario Bros. and Super Mario Bros. 3 game styles. Also, Circling Boo Buddies do not make facial expressions unlike in other games.
A single group of Circling Boo Buddies appears in the Gloomy Woods in Mario & Luigi: Paper Jam, when the player is controlling Luigi after Mario and Paper Mario disappear. They are invisible, hence their movements can only be tracked by their shadows. If Luigi touches one of the Boos, they will appear and scare him, causing him to run away. Additionally, one of the Paper Boos' attacks is to separate their copies and move in a motion similar to that of Circling Boo Buddies.
Although Circling Boo Buddies do not appear in Super Mario 3D Land and New Super Mario Bros. 2, Peepas appear instead, behaving identically. Similarly, in Super Mario Galaxy 2, there are groups of Octoboos which behave just like Circling Boo Buddies.
A similar Boo formation is the Boo Buddy Snake.