Boo Crew

Boo Buddy Swarms are Boos known for forming large groups to attack.

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 indestructible and can only be avoided by the player.

Yoshi's Story
In Yoshi's Story, swarms of miniature Boos called Boo Buddies, Little Boos or Baby Boos  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 from off-screen. If Yoshi eats a Heart Fruit and does a Ground Pound near them, they will turn into Lucky Fruit, much like Shy Guys do. Unlike their Super Mario World counterparts, they do not turn transparent and can be destroyed.

Mario Kart series
Boo Buddy Swarms appear in the Ghost Valley courses in Super Mario Kart, Banshee Boardwalk in Mario Kart 64, and Luigi's Mansion in Mario Kart DS. However, the Boos do not hurt the players.

Paper Mario: Sticker Star
A Boo Buddy Swarm reappears in Paper Mario: Sticker Star.

Puzzle & Dragons: Super Mario Bros. Edition
Boo Buddy Swarms appear in Puzzle & Dragons: Super Mario Bros. Edition as allies with a Dark attribute and sub-attribute under the name Boo Crew. Their skill is "Boo Trick", which lets the player be able to move any orbs anywhere for 6 seconds except in Score Attack, while their awoken skill, "Twin Target", raises their attack and damages two opponents for each attribute the player clears four orbs.

Trivia

 * Boos from Super Mario 64 are called "Baby Boos" in Prima Games coverage.