Aliens

Aliens are beings not from the planet Earth. Although mostly appearing in imagination, they are indeed real and have appeared on several occasions (without the kids noticing). Buster is well-known for believing in them, and several others also do. Their design has varied more than any other character on the show, mostly depending on the episode, if it is real or imagined, and who is imagining them. There is an extremely slight chance that the entire Arthur show is actually acted out by aliens, as seen in the ending of The Boy Who Cried Comet!.

Actual Aliens
Despite most of the characters' belief, aliens are indeed real in the Arthur world. They mostly make cameo appearances. There have been several designs for actual aliens, meaning that there is more than one alien race. They are responsible for stealing D.W.'s snowball in D.W.'s Snow Mystery, which was the only time Buster's theory was correct. Some of them are also ballet fans, as seen in The Chips Are Down.

Buster
Buster is the most prominent alien believer in the whole Elwood City community. He is often quick to blame aliens for little problems happening, mostly in hopes that they will come, so he can meet them. He once thought that, among others, D.W., Harry Mills, and Sue Ellen were aliens in disguise (he only officially changed his mind about the latter two).

Francine
Because none of the kids know what happens in the Teachers' Break Room, Francine theorized that the teachers were really aliens set out to make quizzes impossible to pass. (She later said that she only made the alien theory because she knew Buster eventually would have, and she saved him the trouble of having to say it.) She imagines the aliens with huge heads and a single eye, and that they like eating slugs.

Mr. Morris
When Buster visited him in April 9th, Mr. Morris told Buster a story about when the dogs at a local dog show acted strangely and ran to a nearby field that had crop circles. Although most people thought they were from a tractor, Mr. Morris still was convinced it was from aliens. (Note: Aliens and/or their UFO were never seen, only the crop circles that were assumed to be from them.)

The Tibbles
When the Tibble Twins were pretending to be the Terrific Turbo-Trooper Toy T-Bot Team, they thought D.W. and Emily were space "bad guys", which can be assumed to be aliens.

Harry Mills
Harry seems to also to believe in aliens, as he said to Buster that Mr. Ratburn was definitely "one of them". Mr. Ratburn's random comment about lizards was because they first thought Buster's mom was dating him, and Arthur made up that Buster had a pet lizard that eats rats to discourage him from dating her.

Episode Appearances:

 * Season 1
 * D.W.'s Snow Mystery (imagined and real- Buster though that they stole the snowball, and for the only time ever he is right. The real aliens were the 3-eyed ones instead of the brain ones in Buster's imagination.)
 * Sue Ellen Moves In (imagination- Buster thought Sue Ellen and family were aliens)
 * Arthur's First Sleepover (only mentioned- the kids made fake UFO's)
 * Misfortune Teller (mentioned- in the intro about weird things that people believe, Buster released balloons with his address on them so aliens could find where he lived. When they came, he planned to sell them 'I Love Earth' T-shirts and bumper stickers.)
 * Season 3
 * Attack of the Turbo Tibbles (imagination- the space "bad guys")
 * D.W. Tricks the Tooth Fairy (mentioned- D.W. wished that the UFO from Arthur's First Sleepover was real so her pictures would be worth money and she could buy more clothes for Uni.)
 * The Chips Are Down (real aliens, cameo- blamed for the unusual friendship of D.W. and Binky, but were just ballet fans)
 * Background Blues (mentioned- Buster thought his relatives were taken away by aliens because he couldn't find any information about them)
 * The Ballad of Buster Baxter (mentioned- Buster was afraid that aliens had replaced his friends with cyborg twins "programmed to make him miserable" because they were acting differently around him.)
 * Arthur's Almost Live Not Real Music Festival (cameo- a clip from the end of D.W.'s Snow Mystery represents ice cream in the song Leftovers Goulash)
 * What Scared Sue Ellen? (mentioned- Buster thought the howling was from an "intergalactic creature of some kind")
 * Season 4
 * The Contest (imagination- in Buster's South Park-style T.V. show idea, Arthur was abducted by aliens to be eaten, but was rejected because he was too fat.)
 * The Fright Stuff (imagination- Buster's Halloween prank idea involved recreating a scene from H.G. Wells' The War of the Worlds, but it would be too hard to build, so they went with something simpler.)
 * Season 5
 * Bitzi's Beau (imagination- Buster thought Harry Mills was an alien until he got to know him. Harry thought Mr. Ratburn was an alien because of a misunderstanding.)
 * Season 6
 * The Boy Who Cried Comet! (real, cameo- At the end, all the friends take off their masks and reveal that they are aliens, and the Sugar Bowl was a stage set up on another planet. This either means that all Arthur episodes are played by aliens, but is more likely just this one was.
 * Season 7
 * Elwood City Turns 100! (mentioned- Buster insisted the play includes aliens, to which Brain finally admits there was a possible sighting in the 1950's. They build a wooden UFO, but when it was smashed, Buster improvised the scene by dressing as Dr. Ector, from the planet Corn.)
 * Return of the Snowball (real aliens, cameo- the two young alien brothers that stole the snowball in D.W.'s Snow Mystery' are scolded by their dad for taking without asking. Note: their design is reused from Buster's imagination, not the real ones, from D.W.'s Snow Mystery.)
 * April 9th (mentioned- Mr. Morris tells Buster a tale from his young adult days where dogs at a dog show acted strange and there were crop circles in a nearby field. He believes that it was the work of aliens.)
 * Buster's Amish Mismatch (imagination- Buster imagines racing aliens with a flying hay-bailer, but is disappointed that the Amish donkey-drawn plow cannot fly.)
 * Season 10
 * Feeling Flush (imagination- Buster thought that aliens are making intergalactic rest-stops in Francine's bathroom, but it turned out to be Nemo.)
 * Season 13
 * Looking for Bonnie (imagination- When he plucked the "magic" guitar string, Buster envisioned himself, Arthur, and Francine playing music at Stonehenge. Their performance was interrrupted by a UFO overhead, and an alien holding a gun-shaped object. When they realized it was really a musical instrument, Buster played with "Yeertzel" until the alien's mother called him back for dinner.)
 * The Great MacGrady (mentioned- Buster thought Mrs. MacGrady was kidnapped by aliens to teach them Earth-cooking, but she had cancer and had to stay home for a few weeks.)
 * Season 15
 * Buster the Lounge Lizard (imagination- Francine theorized that the teachers are really aliens, but admits that she only said so to save Buster the trouble, knowing he would eventually say it.)