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). Once, in The Butler Did... What? he said Bailey was an alien from an alien invasion team.

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
 * Arthur's Eyes (imagination)
 * D.W.'s Snow Mystery (imagination and real)
 * Sue Ellen Moves In (imagination)
 * Arthur's First Sleepover (UFOs are mentioned)
 * Misfortune Teller (mentioned)
 * I'm a Poet (UFOs are mentioned in Prelutsky's poem)
 * Season 3
 * Attack of the Turbo Tibbles (imagination)
 * D.W. Tricks the Tooth Fairy (mentioned, referring to "Arthur's First Sleepover")
 * The Chips Are Down (real)
 * Background Blues (mentioned)
 * The Ballad of Buster Baxter (mentioned)
 * Arthur's Almost Live Not Real Music Festival (cameo- a clip from the end of "D.W.'s Snow Mystery" in the song Leftovers Goulash)
 * What Scared Sue Ellen? (mentioned)
 * Season 4
 * The Contest (imagination)
 * The Fright Stuff (imagination)
 * Muffy's New Best Friend (imagination)
 * Season 5
 * Bitzi's Beau (imagination)
 * Kids Are From Earth, Parents Are From Pluto (mentioned)
 * Season 6
 * The Boy Who Cried Comet! (real)
 * Season 7
 * Elwood City Turns 100! (UFOs are mentioned)
 * Return of the Snowball (real)
 * April 9th (UFOs are mentioned)
 * Buster's Amish Mismatch (imagination)
 * Season 8
 * Fernkenstein's Monster (imagination)
 * Season 10
 * Feeling Flush (imagination)
 * Season 11
 * Phony Fern (mentioned in the way Buster describes the cell phone "takeover")
 * Germophobia (mentioned)
 * Season 12
 * Home Sweet Home (mentioned)
 * I Owe You One (imagination)
 * Season 13
 * Looking for Bonnie (imagination)
 * The Great MacGrady (mentioned)
 * MacFrensky (toy)
 * Season 15
 * Buster the Lounge Lizard (imagination)