Big Boos are a large sub-species of Boo. Usually, they behave very similar to regular Boos and maintain a steady resemblance, except that they are bigger in size. Big Boos are also referred to as Atomic Boos in the Paper Mario series.
Super Mario series
Super Mario World
Big Boos first appear in Super Mario World, inhabiting various Ghost Houses around Dinosaur Land. Despite them being about three times the size of regular Boos, these enemies behave in the same mannerism as their smaller counterpart, meaning that they have all the same traits such as fading and hiding behind his arms if looked at, occasionally peeking to see if he is still being watched. However, if Mario or Luigi's back is turned, they can attack them from behind by slowly approaching them.
Normally in the game, Big Boos serve much more difficult to pass than their smaller brethren. As such, Mario or Luigi might be required to use a Springboard to jump over these massive ghosts or use a Spin Jump over Big Boo's head while running fast. Additionally, you may also use a Cape Feather to glide over Big Boos or perform a sliding Cape Attack during descent to defeat the Big Boo. Additionally, in the Secret Donut Plains Ghost House, there is a Big Boo found in a secret room as a hidden mini-boss. Throwing Grab Blocks at him enough times defeats him along with the two Boos he is accompanied with in battle.
Super Mario 64/DS
Big Boo reappears in Super Mario 64 in Big Boo's Haunt where they act in the same manner as regular Boos do so once again. One can be found in the hallway when first going out into the courtyard and it goes down into the courtyard. Mario must find the Big Boo and Ground Pound on it to release the entrance to Big Boo's Haunt which shrinks anyone who enters to fit inside of the area.
There are three more Big Boos found there; one can be found on the top of the balcony on which Mario must defeat in order to gain a Power Star. Another one can be found on the bottom floor of the area, while one more can be found in the first mission in the front entrance when defeating all of the regular Boos in the mansion, excluding the lowest floor. Defeating that Boo summons stairs to come so that Mario is able to collect one of the Power Stars once again.
Big Boos reappear in the DS remake, Super Mario 64 where they act in the same manner as they did in the original version and are once again enemies. King Boo, a Big Boo species is a boss in this game who holds Luigi captive.
New Super Mario Bros. Wii
Big Boos reappear in New Super Mario Bros. Wii where they can be found in various Ghost Houses in the game. They act identically as they did in Super Mario World, but are always with a crowd of regular sized Boos on their side.
Super Mario Galaxy 2
Big Boos make a reappearance in Super Mario Galaxy 2 in Haunty Halls Galaxy in the mission, "Sneaking down the Corridor". They always appear in a group of five with the one in the middle being the biggest.
Super Mario 3D Land
Big Boo reappears in Super Mario 3D Land.
During the game, Big Boo makes an appearance in World 4-4, a Ghost House. This Boo chases down Mario through a small maze of platforms and then some moving platforms. At the end of this path is a Star that Mario can use to defeat the Big Boo.
The Big Boo later reappears in World 5-3, another Ghost House. They appear at the end of the level. Mario must walk along a small tightrope, and upon reaching the middle, a Big Boo appears on the far side. Mario must jump over the Boo to reach the other side with the door to the level's Flagpole.
Big Boo again appears in the level World 8-4, another Ghost House. It appears on the final reversible platform in the level, near the end of its usage.
Super Mario World 2: Yoshi's Island
Big Boo appears in Super Mario World 2: Yoshi's Island as an enemy in the game and appear in certain towers and castles. They are like regular Boos, but are three times bigger than the Boo. They act the same as regular Boos do in this game and can be defeated when Yoshi faces it and shoots an egg at a wall and then at the face. They release five Stars when defeated.
Yoshi's Island DS
Big Boos reappear in Yoshi's Island DS where they once again are enemies found in towers and castles and act the same as they did in the predecessor.
Big Boos reappear in the level Ghost Mansion in Yoshi's Safari. If one is shot by Mario's Super Scope, it causes a wall fall on Big Boo and it splits up into three regular Boos. Mario wins a task if he then shoots the Boo with its tongue sticking out.
Super Mario RPG: Legend of the Seven Stars
Paper Mario series
Paper Mario: The Thousand-Year Door
Big Boo appears in Paper Mario: The Thousand-Year Door where they are now known as Atomic Boos. They can be found in Creepy Steeple and are created by 200 different species of Boos. In the Creepy Steeple, Mario frees 200 Boos. Mario must knock with his Super Hammer and then use his Spin Attack. The Boos then combine all together and form the Atomic Boo and fights Mario and co. in a battle. He attacks Mario with his Glare attack by flinging Boos at him. He then becomes transparent and flies over Mario who can give Mario status aliments if not careful. When Atomic Boo is defeated, he splits into regular Boos as regular enemies. Mario also receives the Lucky Start badge.
Super Paper MarioBig Boos make an appearance in Super Paper Mario as rare enemies found in Merlee's Mansion. They, once again, go by the name Atomic Boo and they have decreased in strength. When Mario faces the opposite direction that Big Boo is facing, Big Boo charges at Mario. Mario can counter this attack by facing Atomic Boo who then faces away from Mario and returns to his regular position. These enemies also, by occasion, appear in the mini-game Mansion Patrol where they are slower than regular Boos, but take multiple hits to defeat.
Paper Mario: Sticker Star
Big Boo appears as a mini-boss in Paper Mario: Sticker Star. It is fought at the end of The Enigmansion after Kamek combines all of Mario's caught Boos in the Book of Sealing. It comes into the battle with 100 HP. The Vacuum Thing Sticker, the Infinijump Stickers, or the Spike Helmet stickers makes the fight with the Big Boo extremely easy. When it turns invisible, use a spray-based thing sticker like the Soda or the Squirt Gun.
Mario Party series
Mario PartyBig Boo has also made several appearances in certain mini-games in this game. A single Big Boo chased a group of characters in the mini-game Running of the Bulb. In this game, the Big Boo was chasing the characters across a dungeon. If a character touches the Big Boo, then they are eliminated from the mini-game. If Big Boo manages to get the light bulb, then every character will lose the game.
Mario Party 2
Big Boo has made numerous appearances in the Mario Party series as a character. It's main role as a non-playable character was in Horror Land in Mario Party 2. In this game, Big Boo appeared in the upper corner of the board behind a locked door that required a Skeleton Key to open. During the daytime, a giant Big Boo statue was behind the door However, during the night, a character can encounter an enormous Boo, who will steal either coins from all players for a fifteen coin fee, or steal a star from all players for a 150 coin fee.
In the mini-game Skateboard Scamper from Mario Party 2, a giant Big Boo chased a group of characters on skateboards across a haunted mansion that was filled with traps. If a Big Boo captured a character in this game, then they were automatically eliminated.
Mario Party 4
The same Big Boo reprises its role in Boo's Haunted Bash in Mario Party 4. In this game, the characters needed to step on a set of happening spaces three times in order to activate the large grandfather clock in front of the Big Boo. The Big Boo in this game functioned exactly the same as the one in Mario Party 2.
Game & Watch Gallery series
Game & Watch Gallery 4
Big Boos reappear in Hotel Mario where they can be found in Ludwig's Thump Castle Hotel as powerful enemies being about 1/6th the size of an entire hotel floor. They act, once again, like regular Boos and as they did in Super Mario World.
Mario Pinball Land
Big Boo appears in Mario Pinball Land as one of the four bosses in the game. He holds the final Star Key and appears in the area, Fun Fair. He can be attacked when Mario hits him from the back until he becomes defeated and Mario gains all four Star Keys.
Super Princess Peach
Big Boo appears in Super Princess Peach where it only appears in World 3-2 and acts the same as always, but can also be permanently stunned by light in this game. These enemies can be used as platforms when stunned and cannot harm Peach in any kind of way. There are also new subspecies in this game called Mad Big Boos whose power is used by the Vibe Scepter.
Luigi's Mansion: Dark Moon
In Luigi's Mansion: Dark Moon, a Big Boo comprised of 10 normal Boos is battled in the Treacherous Mansion in the Train Exhibit. This is not the same Big Boo as Boolossus because Boolossus was made of 15 Boos instead of 10. To defeat it, Luigi must either launch or lure the Big Boo into hitting the front drill of an encircling train, causing him to pop. The ten Boos will all be stunned, and Luigi has to launch all of them into cars on the train.
- Despite Boolossus being made of fifteen Boos and Atomic Boo in Paper Mario: The Thousand-Year Door having two-hundred Boos creating him, Boolossus is considerably bigger.
- Oddly enough, the Big Boo in Super Mario 64 has the high pitched laugh of a normal Boo, but has the low grunt of a Thwomp when he is hit.