Boo Guy

Boo Guys are ghostly enemies from the Super Mario universe, featuring primarily in Yoshi titles. Not your typical Boo, these ghosts wear Shy Guy masks and behave more like Shy Guys than Boos. They hover just above the ground and will move toward the player regardless of whether they are being looked at or not.

Super Mario World 2: Yoshi's Island
Boo Guys can be found in underground or fort stages, fulfilling a variety of roles much like standard Shy Guys. In most cases, they spawn from Warp Pipes and will amble about, only harming the Yoshi if it walks straight into one. In some stages, Boo Guys will handle large spiked balls on chains and attempt to crush Yoshi with it, or can be seen in the background operating traps.

Boo Guys can be defeated with just about any attack. Yoshi can eat them and turn them into eggs, spit them out so they roll into walls or other enemies, or can just jump on them or shoot eggs at them.

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.