Mini Boo

Mini Boos, originally known as Little Boos, Baby Boos  , or simply Boo Buddies , are smaller variants of Boos. Though they have very similar appearances, with the exception of size, Mini Boos are often not used as actual enemies, and when they are, they do not act as Boos. As an item, they may be used to assist the character who summons them.

Yoshi's Story
In Yoshi's Story, swarms of Little 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 Little Boos will stop in their tracks and swoop at Yoshi. It is impossible to defeat all of the Little 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. They resemble Boo Crews from Super Mario World, except their position moving back and forth is more fixed instead of swooping from above.

Mario Power Tennis
In Mario Power Tennis, Boo's Offensive Power Shot, Boo Blast, involves Little Boos. The move summons many Little Boos into a large tennis racket, hitting the ball with it, then allowing them to cloak the ball in hopes of earning a point. As is the case again, opposing players are still able to return the ball if they know where to be.

Super Mario Galaxy
In Super Mario Galaxy, transparent Little Boos can be spotted on the finish line planet of Ghostly Galaxy during the mission A Very Spooky Sprint. They merely come from the planet's surface a short distance before disappearing.

Mario Super Sluggers
In Mario Super Sluggers, Mini Boos appear under name as an item. The Mini Boo item makes the ball invisible to the player in the outfield. They are found in Wario City and are required for King Boo to join the player's team in Challenge Mode.

Trivia

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