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 they are real or imagined
 * if imagined, who is imagining them

There is an extremely slight chance that the entire Arthur show is actually acted out (or at least reenacted) by aliens, as seen in the ending of "The Boy Who Cried Comet!."'

Species

 * 3-Eyed Aliens
 * Yertzel
 * Yertzel's Mother
 * Shnarrrpians
 * Blorrg
 * Martians
 * Aliens (The War of the Worlds)
 * Creepy Cool Mutant Aliens
 * Crab Nebulians
 * Horseshoe
 * Kodo
 * Scary Purple Alien
 * Brainy Aliens

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 mentioned)
 * Misfortune Teller (mentioned)
 * I'm a Poet (UFOs mentioned in Prelutsky's poem)
 * Season 3
 * Attack of the Turbo Tibbles (imagination)
 * D.W. Tricks the Tooth Fairy (UFOs 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 mentioned)
 * Return of the Snowball (real)
 * April 9th (UFOs mentioned)
 * Buster's Amish Mismatch (imagination)
 * Ants in Arthur's Pants (real)
 * Season 8
 * Fernkenstein's Monster (imagination)
 * Season 10
 * Feeling Flush (imagination)
 * Season 11
 * Germophobia (mentioned)
 * Buster Gets Real (mentioned)
 * Season 12
 * Home Sweet Home (mentioned)
 * I Owe You One (imagination)
 * Season 13
 * Looking for Bonnie (imagination)
 * The Great MacGrady (mentioned)
 * MacFrensky (toy)
 * Arthur's Number Nightmare (mentioned)
 * Season 15
 * Buster the Lounge Lizard (imagination)
 * The Butler Did... What? (imagination)
 * Season 16
 * Based on a True Story (mentioned)