From the Super Mario Wiki
Circling Boo Buddies are a group of Boos moving in a circular formation found in Super Mario World. While making different expressions, they chase and circle around Mario and Luigi, slowing down their process. Touching any single one of the Boos that make up a Boo circle can damage the Mario Bros. They only appear in small rooms in Ghost Houses and other haunted areas, including swamps and shipwrecks. Unlike regular Boos, they do not become shy when looked at, suggesting Boos have more confidence in groups.
and Yellow Toad
in an area with the Circling Boo Buddies in New Super Mario Bros. Wii
Circling Boo Buddies also make an appearance in New Super Mario Bros. Wii, in the World 5-, where they act just like they did in Super Mario World.
Circling Boo Buddies, as they appear in New Super Mario Bros. U
These Boo Buddies also appear in New Super Mario Bros. U, where they are found in haunted areas, just like in the older appearances. Their design is almost the same of New Super Mario Bros. Wii ones. In New Super Mario Bros. U, they come in a variety of forms, with different numbers of Boos and making up larger or smaller circles.
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.
Another similar Boo formation is the Boo Buddy Snake.