Boo Guy

A Boo Guy is a ghostly-looking Shy Guy. Boo Guys first appeared in Super Mario World 2: Yoshi's Island. Boo Guys are white and have a wispy tail similar to other Boos. As such, Boo Guys hover rather than walk. As with Shy Guys, Boo Guys wear masks, move slowly, and can be stomped or swallowed by a Yoshi, providing Yoshi Eggs. They can also spawn from Warp Pipes, similarly to Shy Guys.

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, first occurring in Bigger Boo's Fort, either wandering or spawning from Warp Pipes. They can follow the Yoshi for short distances. If the Yoshi collides with one, it drops Baby Mario. To defeat them, the Yoshi can stomp on it or swallow it for eggs. If the Boo Guy is spat out, it can roll through several enemies, defeating them.

In the background of some levels, Boo Guys are seen using harmful traps, including Tekkyū Dosun. If the Boo Guy sees a Yoshi, it drops the mace. Once the mace hits the ground, the Boo Guy then takes time to lift it to perform its attack again. Also, groups of Boo Guys operate a long, revolving spiked mace, rotating at a constant speed.

In some castles, Boo Guys are stationed in long rows in the ceilings and drop bombs. These ones are known as Relay Heihō.

Yoshi's Island DS
Boo Guys reappear in Yoshi's Island DS. Boo Guys remain unchanged, although they are not seen operating machinery nor are they seen relaying bombs.

Yoshi's New Island
Boo Guys return in Yoshi's New Island.

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 hanging paper, 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. They attack by rolling into Mario or Luigi. Before they attack, however, they split into two transparent copies: one real, one fake. Only the real Boo Guy can harm Mario or Luigi. The Boo Guy points to the Mario brother prior to attacking. Ghoul Guys, stronger and faster versions of the Boo Guys, appear in Shroob Castle.

Trivia

 * The Super Mario World 2: Yoshi's Island Player's Guide describes these enemies as "Boo babies", whereas some Japanese guides instead call them Shy Guy ghosts.