From the Super Mario Wiki
Boo Buddy Swarms are Boos known for grouping in masses to attack. They appear in Super Mario World and Yoshi's Story.
Super Mario World
In Super Mario World, they are found only in the Donut Ghost House and Forest Ghost House. They are usually transparent, which is when they are harmless, but periodically will turn opaque, which is when they are dangerous, to dive at Mario. They are indestructable and the only way to deal with them is to run past them and enter another room.
In Yoshi's Story, they appear in Ghost Castle. These tiny ghosts will swoop at Yoshi very rarely, unless he throws an egg at one of the members of the group. Then, all of the Boos will stop in their tracks and swoop at Yoshi. It is impossible to defeat all of the Boos, as more will just appear off-screen. Unlike their Super Mario World counterparts, they don't turn transparent and can be destroyed.
Mario Kart series
A Boo Buddy Swarm appears in Mario Kart DS and Mario Kart 7, in the garden of Luigi's Mansion. However, the Boos don't hurt the players.
Paper Mario: Sticker Star
A Boo Buddy Swarm reappears in Paper Mario: Sticker Star.
- ^ Nintendo Power Advance Volume 4, p. 53