Perplexing Plurals

Part of what you know when you know a language is how to make your words fit into a grammatical system. This can be fairly simple, as in English (the odd -s here, the odd -ed there) or fairly complex, as in the Russian case system. One thing all systems have in common though, is that they work all the time. For every singular, there is a plural. It might be regular (trope, tropes) or irregular (goose, geese), but you know it's there and you know what it is.

...until you find yourself needing to talk about octopuses...octopus...octopi- no, that's Latin...octopodes? Ugh, now this is going to bother me all day.

Sometimes you go for a plural and...there isn't one. Maybe it's an unfamiliar word, maybe you've heard different options and don't know what to pick, maybe language is just screwing with you. Whatever the reason, it feels so overwhelmingly wrong that you have to stop and talk about it. Almost always derails the conversation at hand, and may be the equivalent of a Logic Bomb to a Grammar Nazi.

Truth in Television. Related to the phenomenon of "paradigmatic gapping" in Real Life, where a grammatical form that logically should exist just...doesn't.

Note: This is crucially not a trope used by Funny Foreigners. It is confusion about a speaker's own language (and the need to stop and work it out) and usually not caused by ignorance or Book Dumb (although expect a character beset by Sesquipedalian Loquaciousness to be immune).

Fan Works

 * In Cartographer's Craft, a Harry Potter fanfic by copperbadge, Hermione insists on using "Horcruces" as the plural of "Horcrux" instead of "Horcruxes", to everyone else's chagrin.

Film

 * Mystery Men: Captain Amazing comments to Casanova Frankenstein that they have always been each other's greatest "nemisises..nemisi..." (It's "nemeses".)

Literature
"1. Witches.
 * In Good Omens, Shadwell's instructions to Newt are to search for:

2. Unexplainable Phenomenons. Phenomenatrices. Phenomenice. Things, ye ken well what I mean."


 * It's "phenomena". (Doo doo, doo doo-doo.)


 * Since the climax of The Pyrates occurs at the aptly-named Octopus Rock, the inevitable confusion ensues. Antihero Colonel Blood uses the incorrect Latinate plural "octopi," and smarmy hero Captain Avery offers him the correct (though antiquated) Greek "octopods" or "octopodes." Largely out of contrariness, Blood elects to go with "octopussies."

Live Action TV
"Bobby hands Carol a new plug Carol: Now I hope these are the right plugs. Bobby: Just the kind the teacher said to get instead of that old octopus. Alice: Do you know that all last night I dreamed about octopuses?...Octopussys?...Octopi?"
 * The Brady Bunch: The B Plot of the episode "The Personality Kid" concerns Bobby & Cindy going ape-shit over safety. The following concerns some electrical outlets in the kitchen, which have so many extensions & multi-prongs plugged in that one is referred to as an "octopus."

"GOB: I have a few terms(whistle)... I mean conditions(whistle)... I have one term and one condition."
 * In Buffy the Vampire Slayer, Riley comments that, after falling in with the main characters, he suddenly finds himself needing to know the plural of "apocalypse".
 * In another episode the Trio kept stumbling over the plural of "nemesis." Became a Brick Joke when a vampire the next season said it was "nemeses," which Buffy noted.
 * The girls of iCarly spend a fairly large amount of "iGo to Japan" arguing of the current plural of the word "possum".
 * There is a variant of this in Arrested Development (subverted maybe?). GOB is dense enough that he think he can avoid using an "S" by switching to a different plural noun. GOB has just chipped a tooth and is now whistling his S's:

Music
"I want a hippopotamus for Christmas Only a hippopotamus will do No crocodiles or rhinoceroseses I only like hippopotamuseses And hippopotamuses like me too!"
 * Used in the Novelty Song "I Want a Hippopotomus for Christmas."

"One hippopotami cannot get on a bus Because one hippopotami is two hippopotamus And if you have two goose, that makes one geese A pair of mouse is mice, a pair of moose is meese"
 * The Allan Sherman song "One Hippopotami" (a parody of "What Kind Of Fool Am I?") is half this trope and half "pair" puns.

Tabletop Games

 * The Tyranids in Warhammer 40000 have the Carnifex. Good luck getting the fans to agree on a plural.
 * ...Bad news?
 * In Magic: The Gathering, a powerful mana-generating card is the Mox (Mox Ruby, Mox Sapphire, and so forth). Traditionally, the plural is "Moxen".

Video Games

 * Kingdom of Loathing tends to have a lot of fun with plurals, with "box" becoming "boxen", "kiwi" becoming "kiwus", "fruit basket" becoming "fruits basket", and "liar's pants" becoming "liar's pantses, precious".
 * Nobody in the Free Space fandom is sure what the plural of "Sathanas" is supposed to be (given that it's an ancient name for Satan, it probably doesn't have one). The game itself avoids the issue by always referring to that ship class as a "Sathanas juggernaut".

Western Animation

 * In one episode of Kim Possible Drakken builds a machine that drains a lake, and he comments on the machine not filtering out all the "fishes". This launches an argument between him and Shego about what the plural of fish is. Shego tells him that both fishes and fish are correct plurals for the word.
 * In the Phineas and Ferb episode "Greece Lightning", the narrator of an educational filmstrip is unsure how to refer to platypi. Platypuses. Platypeople?

Jokes/Real Life
"The genitive plural of a noun (used with a numeral to indicate five or more of something, as opposed to the dual, used for two, three, or four, see Russian nouns) is a rather unpredictable form of the Russian noun, and there are a handful of words which even native speakers have trouble producing this form of (either due to rarity or an actual lexical gap). A common example of this is kocherga (fireplace poker). The joke is set in a Soviet factory. Five pokers are to be requisitioned. The correct forms are acquired, but as they are being filled out, a debate arises: what is the genitive plural of kocherga? Is it Kocherg? Kocherieg? Kochergov?... One thing is clear: a form with the wrong genitive plural of kocherga will bring disaster from the typically-pedantic bureaucrats. Finally, an old janitor overhears the commotion, and tells them to send in two separate requisitions: one for two kochergi and another for three kochergi. In some versions, they send in a request for 4 kochergi and one extra to find out the correct word, only to receive back "here are your 4 kochergi and one extra.""
 * Some Russian jokes deal with the unpredictable nature of Russian plural nouns. An example from Wikipedia:


 * There's an old Italian joke where a man doesn't know if the plural for "Belga" (a person from Belgium) is "Belghi" or "Belgi". He eventually resorts to write down "Un Belga. Anzi due!" (One Belgian. I mean, two.)
 * Brian Regan has an entire routine built around this trope. It starts with "boxen" (like oxen) of donuts and just snowballs from there.
 * Richard Lederer's "Foxen in the Henhice" is based around the same concept.
 * There's a joke about someone shipping a pair of mongooses to a zoo and being unsure of what the plural of "mongoose" is. They start out writing a note referring them to "mongooses", then try "mongeese", and eventually resort to writing "Enclosed is the mongoose you ordered. Also enclosed is the other mongoose you ordered".