From the Super Mario Wiki
Big Boos, also called Atomic Boos or Mega Boos, are a sub-species of Boo. Usually, they resemble and behave similarly to normal Boos, except that they are larger.
Super Mario series
Super Mario World
The Big Boos (or The Big “Boo”) first appear in Super Mario World. In the game, Big Boos inhabit the various Ghost Houses located around Dinosaur Land. Curiously, although they are over ten times bigger than a normal Boo, Big Boos still have the same mannerisms of their smaller counterparts. Of course, this means that when looked at, a Big Boo will fade and hide behind its arms, occasionally peeking to see if they are still being looked at. However, if someone's back is turned, a Big Boo will attack them from behind.
In the game, Big Boos are much more difficult to pass than their smaller brethren. As such, Mario or Luigi usually have to use a trampoline to jump over these massive ghosts. However, if Mario or Luigi can get a running start, they can usually Spin Jump over a Big Boo. Additionally, if there is room, the brothers can use a cape to fly over a Big Boo or perform a sliding Cape Attack during descent to actually destroy the Big Boo. Additionally, a translucent white one also called "The Big Boo" is a hidden boss of the Secret Donut Plains Ghost House. He can be defeated by throwing Grab Blocks at his face. He has two normal Boos accompany him in battle.
Super Mario 64/Super Mario 64 DS
“Ghosts...don't...die! Heh heh heh! Can you get out of here......alive?”
Big Boos return in the game Super Mario 64. Again, a Big Boo disappears when looked at, and become visible again when Mario turns away. They have a high pitch laugh like a normal Boo, but make the same low pitch grunt as a Thwomp does when hit.
Big Boos can only be attacked when visible, so Mario has to attack these massive ghosts by either Ground Pounding them from above or punching them from the side. Inside Big Boo's Haunt, Mario can fight three Big Boos. These Big Boos take three hits to defeat, becoming smaller after each hit. After destroying a Big Boo, Mario will receive a Power Star. In the Japanese version, the giant Boos are known as "Boss Boo" rather than their usual Japanese name "Atomic Boo". The giant Boo that appears when all the ghost house Boos are defeated is also known as "Master Boo".
In the DS remake, Super Mario 64 DS, King Boo makes an appearance. In this game, King Boo has his trademark crown from later appearances and characteristic cackle. However, in-game, he is erroneously referred to as Big Boo in English versions.
New Super Mario Bros. Wii
Big Boos reappear in the game New Super Mario Bros. Wii, and act much like they do in Super Mario World. They can only be encountered in Ghost Houses, and are usually seen with many smaller Boos. Unlike regular Boos, a Big Boo will not stop attacking unless every player is facing it, unless there are 4 players, in which case only 3 need to face it.
Super Mario Galaxy 2
In Super Mario Galaxy 2, Big Boos are known as Mega Boos and follow Mario through a thin hall, not stopping unless they are looked at or the player reaches a Power Star or Launch Star. Big Boos are only found in groups of five, with the Big Boo in the middle being larger than the ones surrounding it. Big Boos are only seen in 'Sneaking Down the Creepy Corridor' in Haunty Halls Galaxy.
Super Mario 3D Land
These gigantic Boos reappear in Super Mario 3D Land, where they behave similarly to their New Super Mario Bros. Wii counterparts. They appear in a few ghost houses. Sometimes, if Mario is very far away from the Big Boo and looks in the opposite direction, it will float away instead of drifting towards him.
New Super Mario Bros. 2
Big Boos appear in New Super Mario Bros. 2. Appearing in Ghost Houses, Big Boos can be turned golden by hitting them with a gold fireball or passing through a Gold Ring. While in their golden form, they cover their faces and leave a trail of coins behind them for Mario to take. Whenever it covers its face, the Big Boo glows red, while it radiates a dark shadow around it once Mario stops staring at it.
New Super Mario Bros. U
Big Boos reappear on the Nintendo Wii U title New Super Mario Bros. U, once again only appearing in Ghost Houses. Nothing has changed in their behavior, but now if Mario is watching, Big Boos began to sweat.
Super Mario 3D World
Big Boos also appear in Super Mario 3D World with the same behavior of Super Mario 3D Land. Some can be seen outside the windows in Ghost Houses watching the player, but these Big Boos do not do anything special. Big Boos can now be defeated by Light Boxes, leaving coins.
Super Mario World television series
Big Boos appeared in two episodes of the Super Mario World animated series. In the episode "Ghosts 'R' Us", Yoshi, while exploring Wizardheimer's Haunted House, encounters a Big Boo and three regular Boos. Terrified of the Boos, Yoshi proceeds to run away in a panic as the four ghosts give chase. After running some time through Wizardheimer's Haunted House and losing the three regular Boos, Yoshi manages to trick the Big Boo into entering a vacant room. With the Big Boo apparently trapped in this room, Yoshi locks the door. Unfortunately, the Big Boo simply phases through the wall and resumes chasing Yoshi. Yoshi eventually escapes his Big Boo tormentor by fleeing into the basement of Wizardheimer's Haunted House.
In Super Mario World's final episode, "Mama Luigi", a Big Boo is seen acting as a guard to Neon Castle. Mario, Luigi and Yoshi easily sneak past this Big Boo guard by using a nearby Warp Tube, which led to the basement of Neon Castle.
Super Mario Adventures
An unnamed Big Boo was also featured prominently in the Super Mario Adventures comic by Nintendo Power. After being lured into a Ghost House, Mario and Luigi, after an encounter with several dozen Boos, encounter a Big Boo. Attempting to stare down this Big Boo and the swarm of Boos, Mario and Luigi become exhausted after hours of simply staring at the shy ghosts. Nearly collapsing of exhaustion, Mario spots a door and, dragging Luigi, dashes to it, with the Boos and Big Boos after him. Bursting through the door of the room Mario and Luigi just entered, the Big Boo and his Boo minions are surprised to see Mario dressed as a doctor, with Luigi as his nurse. Acting as a psychiatrist, Mario manages to learn that, as a child, this Big Boo was constantly bullied, which led to its vicious behavior. After hearing this Big Boo's sad tale, Mario gives some helpful words of wisdom and advice. The Big Boo, having become a changed ghost through Mario's therapy, happily leads Mario out of the Ghost House and gratefully thanks him as Mario, Luigi and Yoshi take-off to Bowser's Castle. Another Big Boo was later seen among the attendees to Bowser's wedding to the brainwashed Princess Toadstool.
The Big Boo appears as the boss of the level Ghost mansion in Yoshi's Safari, Mario must shoot it with the Super Scope as he makes parts of the wall fall down on him. Later the Boo will separate into three smaller Boos. Mario must shoot the one with his tongue out to win.
Super Mario World 2: Yoshi's Island / Yoshi's Island: Super Mario Advance 3
In Super Mario World 2: Yoshi's Island, they appear in many Towers and Castles. However, in this game, they are only about three times bigger than a normal Boo, unlike in Super Mario World. Like normal Boos, they could only be defeated by looking away from the ghost and bouncing an egg off a wall, hitting the ghost in the face. They release Stars when defeated. Additionally, one of the bosses, Bigger Boo, starts as a normal Big Boo before being transformed by Kamek's magic.
Yoshi's Island DS
In Hotel Mario, Big Boos, first appearing in Ludwig's Thump Castle Hotel, are powerful enemies, being about one-sixth the size of a hotel floor; Big Boos in this game attacked in a manner similar to the ones of Super Mario World, slowly floating towards Mario or Luigi when they look away, turning transparent when directly looked at.
Super Mario RPG: Legend of the Seven Stars
Mario Party series
Mario Party 2
In Mario Party 2 a Big Boo appears in the Horror Land stage. As a typical Boo, it is able to steal objects from player's rivals, being coins or stars. However, Big Boo will deal for stealing the coins or the stars from all the opponents, while paying to him three times the amount of coins than a normal Boo would do. Characters can ask for him only in night time; in daytime, Big Boo appears as a giant Boo statue.
Mario Party 4
Game & Watch Gallery series
Big Boo is an opponent in the Boxing game for Game & Watch Gallery 4. He is Luigi's second opponent. However, instead of boxing gloves, Big Boo uses smaller Boos to pummel his opponent.
Paper Mario series
Paper Mario: The Thousand-Year Door
Big Boo reappears in Paper Mario: The Thousand-Year Door, now under the retranslated name Atomic Boo. The Atomic Boo is an optional mini-boss composed of approximately two hundred different Boos. To fight this boss, Mario must go to the Creepy Steeple after freeing the 200 Boos in Chapter 4 (if he guesses the number right, a Boo will give him an Ultra Shroom). Even though Mario agreed to be nice to the Boos, they swarm him as soon as they are free. If Mario knocks the Boos away twice with the Super Hammer by using the Spin Attack, the Boos will combine together to make the Atomic Boo.
Atomic Boo attacks by flinging Boos at Mario, charging up a powerful glare attack, hiding his face and becoming invisible like regular Boos, floating over and falling onto both Mario and his partner, and inflicting Mario with status ailments such as confusion and paralysis. Once Atomic Boo is defeated, he will relinquish his Lucky Start badge to Mario and the Boos will stop their swarming and appear as normal enemies.
A purple variant of Atomic Boo, called Dark Atomic Boo, was found in the coding of Paper Mario: The Thousand-Year Door, but cannot be encountered. It would have been fought on the 50th floor of the Pit of 100 Trials. It is reasonable to assume that Dark Atomic Boo would have been a more powerful version of Atomic Boo.
Super Paper Mario
Atomic Boos are rare enemies in Super Paper Mario. In fact, only one appears in Merlee's Mansion, where it is considerably weaker. This Atomic Boo will attempt to attack the player by charging at them while they are facing the direction opposite of them and will turn intangible and shy away if the player faces it. Also in Super Paper Mario, Atomic Boos will occasionally appear in the minigame Mansion Patrol; though slower than regular Boos, Atomic Boos take several hits to defeat instead a single one.
Paper Mario: Sticker Star
In Paper Mario Sticker Star the Atomic Boo is known as Big Boo. In this game, the Big Boo looks like a large, crumpled paper ball with a Boo face. The Big Boo only appears once in the entire game and is a mini-boss found in World 4. It is created when Kamek combines the Boos found in The Enigmansion.
In battle, the Big Boo's only attack is eating Mario and then spitting him out. However, once the Big Boo uses its turn, the candles will come on and the Big Boo will turn invisible. During this time, Mario is unable to attack the Big Boo, but the Big Boo is still able to hurt Mario. In order to make the mini-boss visible again, a Thing sticker with wind or water properties needs to be used in order to extinguish the candles. However, using a Thing sticker with fire properties such as Matches and Lighter while the candle has been put out will light up the candle once more. Note that although the Big Boo is invisible while the candle is lit, it can still take damage from Spike Helmet or Poison Mushroom.
Once the Big Boo has been defeated, the Book of Sealing will stay on its pedestal and the mansion will be free of ghosts.
Mario Pinball Land
In Mario Pinball Land, Big Boo is one of the four bosses. He appears in the haunted house at the Fun Fair. He has the last Star Key. To defeat him, Mario has to attack him from behind. He has no direct attacks, but instead approaches Mario continuously. If Mario comes into contact with Big Boo from the front, he will temporarily spin out of control as if in a daze.
Super Princess Peach
Big Boos also appear in Super Princess Peach. Unlike normal Boos, they are invincible. However, they can be stunned permanently by leading one to a ray of light, which only appears in World 3-2. When they are stunned, they are harmless and can be used to step on to reach a platform. This game also introduces a new Big Boos sub-species, the Mad Big Boo. A Mad Big Boo is a Big Boo under the Vibe Scepter's power. Like normal Mad Boos, they stay still when Peach turns away from one, and chases after her when she looks at it.
Luigi's Mansion Series
Luigi's Mansion: Dark Moon
In Luigi's Mansion: Dark Moon, Big Boo is one of the three bosses fought in Treacherous Mansion, the final mansion in the game, with the final one being King Boo himself. Luigi and E. Gadd were tricked by King Boo to come into the miniature train station inside the Train Exhibit, because E. Gadd thought that King Boo was there along with Mario's painting. Ten Boos are inside a mini train display in which they come together to form into Big Boo. The 10 Boos that assemble together are the only ghosts in the game who are not sucked into the Poltergust 5000, as they are trapped in the trains' cages and later placed in the Boo Canister by E. Gadd.
In battle, the Big Boo has three attacks. Its first attack involves it trying to crush Luigi from above. It will attempt to crush Luigi three times in a row. If it misses all three times, it will be momentarily dazed. Its other attack involves hiding halfway in the ground and attempting to charge into Luigi from there. If it misses Luigi and hits the moving train, it will be stunned. However, if it misses Luigi and the train, it will just float back into the arena. When separated into individual Boos, they can ram into Luigi. The third attack happens only if Luigi pulls back on the tongue for too long. The Big Boo will get out of the stun and ram Luigi. This also applies for the Boos.
To defeat Big Boo, Luigi has two different methods of attacking it. The first one involves avoiding Big Boo while it slams into the ground three times before becoming stunned. Luigi has to pull Big Boo's tongue and launch it into the drill of the moving toy train. The other way is to make Big Boo dash slightly in front of the train. When the Big Boo hits the drill on the train, he splits into ten Boos. Luigi must pull their tongues and send them flying towards the carriages of the train. When one of them hits a carriage, that Boo gets imprisoned inside the carriage and will be unable to escape. Once all of Boos have been imprisoned in the carriages, the mission is complete and the Pixelator takes Luigi out of the train display and back to the Bunker. Afterwards, E. Gadd Pixelates the Boos into the Boo Canister.
Captain Toad: Treasure Tracker
Big Boos reappear in Captain Toad: Treasure Tracker with only a few tweaks from Super Mario 3D World. While they act the same by following a Toad when they aren't being watched, they are far less a threat due to Captain Toad and Toadette headlamp, which can destroy them. To compensate for this, Big Boos generally appear in groups of two or more.
Notable Big Boos
Names in other languages