Boo Guy

Boo Guys are ghostly-looking Shy Guys that first appear in Super Mario World 2: Yoshi's Island. They are white and wear the same masks as other Shy Guys, and have a wispy tail similar to Boos.

Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Boo Guys first appear in Super Mario World 2: Yoshi's Island, and are first found in Bigger Boo's Fort. As with Shy Guys, Boo Guys hover around slowly, and can also spawn from Warp Pipes; colliding with one will cause Yoshi to drop Baby Mario. To defeat a Boo Guy, the Yoshi can stomp on it or swallow it to make Yoshi Eggs. The Boo Guy can also be spat out causing it to roll on the ground and destroy any enemies in its path until it falls offscreen or collides with a wall.

In the background of some levels, particularly fort and castle levels, there are harmful traps that are handled by Boo Guys; the Boo Guys themselves are isolated within confined spaces in walls. Boo Guys that handle a mace (Tekkyū Dosun) will drop it on the ground periodically. Once the mace hits the ground, the Boo Guy then takes time to pull it back up, dropping it again once it reaches the top. Other Boo Guys push a wheel in groups to operate a long, revolving spiked ball. Sometimes, Boo Guys are stationed in long rows in the ceilings and pass bombs from one to another, dropping them on Yoshi when he is underneath (Relay Heihō).

Yoshi's Island DS
Boo Guys reappear in Yoshi's Island DS. Though Boo Guys remain unchanged, only the regular variants appear.

Yoshi's New Island
Boo Guys return in Yoshi's New Island, where they have multiple roles as in their original appearance. Boo Guys that operate traps are seen early in the game and mostly appear in fort and castle-based courses. However, individual Boo Guys that act as enemies only appear in the level Crumble Rock Rumble.

Yoshi's Woolly World / Poochy & Yoshi's Woolly World
Boo Guys return in Yoshi's Woolly World and Poochy & Yoshi's Woolly World and can be eaten by Yoshi, forming a white yarn ball. In the level Spooky Scraps! Don't Get Spooked!, Boo Guys turn into an evil shadow when behind the spooky scraps, which means they can chase Yoshi and cannot be eaten.

Mario & Luigi: Partners in Time
Boo Guys appear in Mario & Luigi: Partners in Time. They are found in Toadwood Forest. In battle, Boo Guys attack by splitting into two transparent copies: one is real, and the other is fake, and only the real Boo Guy can harm Mario or Luigi. They attack by rolling into Mario or Luigi, pointing at the target brother prior to attacking.

Ghoul Guys, stronger and faster versions of the Boo Guys, appear in Shroob Castle.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: Basically Boo babies, Boo Guys can be found wherever the Boo family parties down.
 * Shogakukan guide:「食べてから吐き出すと、コウラのようにどこまでも転がってゆく. 」 ("After you eat it and spit it out, it rolls away like a shell.")

Yoshi's Island: Super Mario Advance 3

 * Shogakukan guide:「食べてからはき出せば、コロコロ転がって敵をまとめて倒せる. 天井に並んでバケツリレーのように爆弾を運び投下するものもいる. 」 ("If you eat it and then spit it out, it will roll around and take down enemies en masse. Some line up on the ceiling and carry and drop bombs like a bucket relay.")

Trivia

 * The Super Mario World 2: Yoshi's Island Player's Guide describes these enemies as "basically Boo babies", whereas at least one Japanese guide instead calls them Shy Guy ghosts.
 * A Boo Guy appears in the concept art of Super Mario Odyssey, suggesting they may have been planned to appear in the game somewhere early in development.