A Boo Guy is a type of Shy Guy ghost that first appeared in the game Super Mario World 2: Yoshi's Island. As its name suggests, Boo Guys are undead Shy Guys that somewhat resemble a cross between a Boo and a Shy Guy. These ghosts greatly resemble the average Shy Guy, although Boo Guys are completely pale and lack feet. Instead of feet, Boo Guys have a wispy tail similar to a Boo. As such, Boo Guys have the ability to hover just above the ground in order to move around. Presumably, Boo Guys are the spirits of deceased Shy Guys, although it is possible these ghosts are created independently of a creature’s death (as Boos themselves may not be the ghosts of the deceased, but simply an undead species capable of an unknown means of reproduction). Like Shy Guys, Boo Guys are individually weak, but can prove to be a threat in large numbers, especially since Boo Guys have shown the ability to organize and strategize with others of their kind, as well as operate advanced machinery.
 Yoshi series
 Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
Like their corporeal counterparts, Boo Guys were allied with Kamek and Baby Bowser’s early Koopa Troop during the events of Super Mario World 2: Yoshi's Island. As such, these ghosts attempted to damage members of the Yoshi clan in order to give Kamek’s Toadies the opportunity to capture Baby Mario. Like Shy Guys, Boo Guys emerged from various pipes located around Yoshi’s Island. They wandered around back and forth to attack the dinosaurs, as did their living counterparts. Boo Guys could easily be destroyed by any attack, including the common stomp attack. Since Boo Guys often continuously emerged from pipes, the Yoshis could eat and swallow the ghosts to refill their egg supplies. Some Boo Guys were stationed inside castles, operating mechanical traps designed to stop Yoshi and his friends. These Boo Guys hid in the walls and ceilings of the castle, and thus could not be defeated by any attack. Regular Boo Guys have two attacking differences from their living counterparts. The first is that they sometimes follow the player for short distances. Second, when spit out, they plow through fellow Boo Guys instead of just one collision and both Shy Guys die.
Boo Guys employed a variety of traps in the game, including maces. Individual Boo Guys operated a single mace attached to a long chain. When a Boo Guy spotted a Yoshi, the ghost would drop the mace from the ceiling, hoping to crush the dinosaur. However, the Yoshi would have to be directly under the mace for the attack to be effective, and the Boo Guys tended to simply drop their maces continuously, instead of dropping the weapon at the most opportune moment. Since the weapon was heavy, it would take some time for the Boo Guy to pull the ball up to attempt another attack, allowing the Yoshis to pass safely underneath the weapon. This was the only way to get past these weapons, as the chain itself would actually damage the heroic dinosaurs. Groups of Boo Guys could operate a much more menacing device, a long, spiked mace that revolved in a circular path. Again, timing was key. Since the Boo Guys operated the weapon at a constant pace, the dinosaurs could time their jumps to pass over or under the dangerous spikes. Lastly, some Boo Guys were stationed in long rows in the ceilings of castles. The chain of ghosts would pass a bomb from one Boo Guy to the next, dropping the bomb when a Yoshi came close by underneath. The Boo Guys would only pass one bomb at a time, and would only bring out a new weapon when the previous bomb had exploded. With fancy footwork, the dinosaurs could avoid the bomb, although at times this proved to be difficult. A Yoshi could perform a Ground Pound to force the Boo Guy currently holding the bomb to accidentally drop the weapon, causing the explosive to detonate on the ground below. This would also visibly shock all the Boo Guys in the chain. After dropping the bomb, the unfortunate Boo Guy would eventually hang its head down in shame, while the other ghosts shook their heads in disappointment. This act would give the Yoshis enough time to pass underneath the row of Boo Guys before they grabbed their next bomb and began passing the explosive down the line.
 Yoshi's Island DS
The Boo Guys reappeared in the game Yoshi's Island DS. Boo Guys remained relatively unchanged from their SMW2 versions. However, in this game, Boo Guys were allied with King Bowser, the grown-up Baby Bowser, who had traveled back in time to the Yoshi’s Island of the past in order to capture the Star Children and use their power to take over the world. As before, Boo Guys attacked members of the Yoshi clan to make the dinosaurs drop their babies, be it Baby Mario, Baby Peach, Baby Donkey Kong, Baby Wario, or Baby Bowser (who the Boo Guys coincidentally worked for just prior to the events of this game). Once dropped, the future Kamek’s Toadies would attempt to capture the crying Star Child and bring it before King Bowser. It is unknown if these Boo Guys traveled with Bowser from the future, or existed in the past and allied themselves with the Koopa king when he arrived in their time.
 Mario & Luigi series
 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.
 Luigi's Mansion
In Luigi's Mansion, Shy Guys reappeared as ghosts with pitchforks (likely a reference to Beezos). To defeat them, Luigi had to suck up the Shy Guy's mask before he could suck it into his vacuum. This is the only type of Shy Guy that has been seen with its mask off. Also, even with the mask off, only its eyes are visible. These are not known as Boo Guys - rather, they are Shy Guy ghosts, which is closer to their Japanese name.
 Close Relatives
 Names in Other Languages