The Duck Multiverse Theory

The Duck Multiverse Theory is a theory developed by ShadeTheNarwhal and SJ Games. This theory connects the universes of numerous fictions, via alternate universe versions of Donald Duck.

Universes

 * Earth-1: Gravity Falls, Mickey Mouse and Marvel Comics all take place within this universe. Donald Duck has two split personalities: Howard the Duck and Ducktective.
 * Earth-2: Looney Toons takes place within this universe. Daffy Duck is an alternate version of Donald Duck.
 * Earth-3: A replica of Earth-1 with one difference: All individuals are genderbent. Daisy Duck is a princess who stumbled across Earth-1 and fell in love with her male counterpart Donald.
 * Earth-4: DuckTales takes place within this universe. Scrooge McDuck is a clone of the deceased Donald Duck of Earth-1 sent into the past of Earth-4. This was before Donald Duck was revived by Bill Cipher during Weirdmageddon.
 * Earth-5: Phineas & Ferb takes place within this universe. A scientific experiment turned Donald Duck, a spy in this universe, into Perry the Platypus. It is theorized that Dr. Doofenshmirtz was involved with the experiment.
 * Earth-6: The YouTuber Berd existed within this universe. Berd is part of the alternate version of Donald Duck's species. This universe has only two dimensions: Length and height. Bill Cipher originated from this dimension and, as confirmed in Weirdmageddon Part 3, destroyed it.
 * Earth-7: Undertale takes place within this universe. The Bird That Takes You Over A Disproportionally Small Gap was an alternate universe version of Donald. After he and his species were exiled to the Underground, Bird became a depressed alcoholic. This caused his wife, Daisy, to divorce him. He was later abducted by Gaster and had his Determination extracted from him, in order to fuel his experiment with Jevil. Bird's lack of Determination made him have zero willpower, making his only action carrying people over the gap in Waterfall.
 * Earth-8: Deltarune takes place within this universe. The Thrash Machine, if customized to be a duck, has Earth-7 Donald Duck's soul inside of it.
 * Earth-9: EarthBound takes place within this universe. Donald Duck was turned into a Spiteful Crow during Giygas'  attack. Unfortunately, he is not turned back upon Giygas' defeat. After defeating Giygas, Ness is dimensionally transported to Earth-7, becoming a skeleton with a blue hoodie. Paula, Jeff and Poo do not survive.
 * Earth-10: Real life takes place within this universe. This universe is often called the Human World. Humans actually came from Jupiter and attempted to survive by sending a UFO to Earth, after one of their weather machines malfunctioned and became an infinite red hurricane. The UFO reached Earth but crashed, making the engine explode. The explosion caused a special type of radiation to flood the Earth, turning all species (including Humans and the then sentient species, Ducks) into non-sentient.
 * Earth-11: Steven Universe and Jaiden Animations take place within this universe. As confirmed by Jaiden, ducks and dogs are the same species. Therefore, we can conclude that Dogcopter is an alternate version of Donald Duck.
 * Earth-12: Not much is known about this universe, although it is known that a version of Donald Duck does exist.
 * Earth-13: DC takes place within this universe. One of Robin's (Damian Wayne, specifically) Birdarangs is sentient after contact with the supervillain Kilg%re, calling itself donaldduck.exe but unable to communicate with humans.
 * Earth-14: Pengu takes place within this universe. Penguins are birds, therefore, we can conclude that Pengu is an alternate version of Donald Duck.
 * Earth-15: Adventure Time takes place within this universe. Penguins are birds, therefore, we can conclude that Gunter is an alternate version of Donald Duck.
 * Earth-16: Pokémon takes place within this universe. All duck-based Pokémon are telepathically linked and worship a supreme duck god named Donaldo. These Pokémon include Psyduck, Golduck, Farfetch'd, Porygon2, Ducklett and Swanna.
 * Earth-17: Minecraft takes place within this universe. The first Chicken that one encounters in the game is an alternate version of Donald Duck. Legend has it that Thanos is actually in the game, represented by his human form Herobrine.

Trivia

 * Sans is Ness.
 * The theory was created when Shade and SJ were talking during a class that they were both in. Shade mentioned the character Howard the Duck, to which SJ commented that he was similar to Donald Duck. It all went downhill from there.