In The Smurfs, the word "smurf" is applied to anything the writers feel would be funny, but especially as a rather transparent substitute for expletives; in some cases, they are then upbraided by another character for using "such language."
Likewise The Snorks and "snork." Only they don't do it quite as much but there have been allusions to the "F-Word", with them saying "Snork off!".
Family Guy once parodied this by having Stewie watching an episode of the Smurfs where one was describing to another his date with Smurfette, using "smurf" to stand in for a lot of naughty words.
South Park once parodied this trope in the episode "Starvin' Marvin in Space", where the boys encounter a race of aliens known as Marklar. The word "marklar" is the only noun in the language, and is used for everything.
Also parodied in Rick and Morty by Squanchy, who uses "squanch" for a LOT of verbs.
Transformers is famous for this, particularly Beast Wars. Various permutations of the word "Slag" are the most popular, but are others, including the perennially popular "kiss my skid plate" (though "skidplate" is used similarly in Animated)
Slag is molten gunk cast off during various metallurgical processes, so it is an obvious stand-in for excrement when dealing with robots. Less fortunate is the Generation One Transformer named Slag, who has retroactively received a very foul name. Some feel that this fits Slag's character a little too well, and the whole thing's an infrequently used running gag among the fandom. They were even going to call his counterpart in Animated Slag, but decided against it.
Perhaps slightly Older Than They Think, with Galvatron in the 1987 season of the first cartoon shouting "Die, you worthless piece of slag!" Given that in Australian and British English, this word means "slut" (or sometimes, in Australia, "spit"), that probably wasn't the best choice, and is why they changed the name in Animated.
Indeed, someone at ITV took so much offence at the term "slag", that all references to it were more or less edited out of Beast Wars when it was aired in the UK.
In one episode of Animated they make a lampshading of this. After Sari expresses surprise that Scrapper has named said counterpart 'Snarl' (taken from another G1 Dinobot), Scrapper replies "Well I was gonna name him 'Slag', but he seemed to take offense at it."
Animated gave us a couple, such as "glitch-head". They're actually seeing how far they could go (they had to cut the line "What the forklift was that"; in the finished episode, the line largely survives, but "forklift" had to be replaced with "front-end loader"), as they weren't allowed to use many initially. However, during the second season, some of the bots (especially Bumblebee) got surprisingly rust-mouthed.
In Prime, the go-to word seems to be "scrap", with Arcee getting the most mileage out of it.
In another case, The Maximals thought they would be returning to Cybertron. Upon remembering that Lawful Good Silverbolt never knew Cybertron, and seems upset about leaving Earth (he's really Dating Catwoman behind everyone's backs and isn't pleased with the fact that they will not be able to see each other upon leaving Earth), Cheetor offers to take him to "Six Lasers over Cybertron". Rattrap instead offers to take him to a little hole in the wall he knows of where the waitresses don't have chest plates and you can see their chassis. Given Silverbolt's personality and his real concerns, this Crosses the Line Twice.
For a while, the word "Slag" spread to other children's action series, such as Batman Beyond.
Parodied on Futurama, where Amy uses made-up equivalents for very mild oaths indeed (such as "Gleesh" for "Gosh", probably a direct parody of "frack" and the like). As well, one episode had the planet of Amazonia, where "snuu-snuu" is used to refer to sex.
Also, Bender frequently yells things like "Cheese it!" and "We're boned."
This may lean more toward simply being anachronistic slang, at least in the case of "cheese it," which was common slang in the 1950's, generally meaning "stop it" or "run away," as a warning when an authority figure was coming near.
Futurama also has Leela using Band names as curses, such as "This Toads the Wet Sprocket!" and "This Wangs Chung!" "So musicians really Roger your Hammerstein, eh?"
One episode in which the gang thwarts Richard Nixon, Nixon shouts out the exact words, "Oh expletive deleted!" This refers to the phrase used to replace Nixon's swearing in transcripts of his recorded conversations with his staff. It became so widely known at the time that some protesters at the White House were even seen carrying signs saying, "IMPEACH THE (EXPLETIVE DELETED)!"
Also, in one episode, about Bender becoming a girl and dating Calculon:
What if he tries to, uh... "Barry White"?
Then there's the fact that Lrrr (Ruler of the Planet Omicron Persei 8!) referred to Fry's penis as his "lower horn."
Kif uses “Oh, monkey-trumpets!” at least once.
Recess used a fictional curse-word "whomp", which seemed to mean "suck" from context. This was lampshaded in an episode when the teachers banned the word, insisting that it must have some kind of hidden offensive meaning. The kids eventually ended up in court in their quest to prove that it was just a word they'd invented.
They intended to use that word for the name of One Saturday Morning's weekday afternoon counterpart, which they were originally naming Whomptastic. But perhaps because using a word that apparently meant "suck" didn't make much sense in that context, they replaced it with the more fitting name of One Too.
In Pepper Ann, "Fuzzy" (the name of a cartoon character within the show's world) was used as a catch-all euphemism ("What the Fuzzy?" "For the love of Fuzzy..." etc.)
Pirates of Dark Water used a set of fictional curse-words, such as "noi jitat" and "jungo-lungo", to bypass the censors, as well as to enrich the sense of an alternate universe. This allowed the characters to retain their foul-mouthed pirate personalities (though most of the protagonists weren't actually pirates) while keeping the show safe for children.
SpongeBob and his friend Patrick use "Tartar Sauce", "Fish Sticks", "Fish Paste", and "Barnacles" as their favored exclamations. One would guess that, to a fish, tartar sauce would be pretty shocking. "What the shell" and "What the halibut" crop up occasionally, too.
The word barnacles seems to be their equivalent of sh* t or bull sh* t as it's sometimes used like "this is a load of barnacles" and "you're full of barnacles", in another example Mr. Krabs had given on ever being able to do something and Spongebob says "Barnacles!" and Mr. Krabs says "Spongebob!" as if he said a very bad word and Spongebob says "Sorry about the foul language, Mr. Krabs". This euphemism is actually directly lampshaded by the song "Barnacles" in the music CD "Spongebob On The High Seas," in which Spongebob, Patrick, Sandy, Mr. Krabs, and Plankton flat-out state that it's a euphemism for curse words. This goes so far as to break the fourth wall and may even count as a Take That! at censorship in general. However, the fact that it encourages censorship by use of this euphemism suggests otherwise.
Chorus: "Barnacles is the way we say what they say we can't say."
Also in the song, Mr. Krabs says he "kissed his assets goodbye" after losing money in the stock market.
Plankton has the habit of saying "What the Davy...?" since "Davy Jones's Locker" is portrayed as the series' equivalent of Hell.
Mr. Krabs is fond of saying "Mother of pearl!" Interestingly enough, he's the father of Pearl. Possibly Mrs. Krabs and Slag (see above) should form a support group for people who have become Unusual Euphemisms.
It seems to be more like a variation "Mother of God".
In "Wet Painters", Spongebob exclaimed "Flapping Flotsam!" when he saw that he and Patrick got a drop of permanent paint on Mr. Krabs's first dollar.
They also use "Captain's Quarters" to refer to butts on occasion.
Spongebob's classic "Holy Krabby Patties!" Hilariously drawn out so you can imagine he's actually saying "holy crap" for as long as possible.
There are also a lot of episodes where the characters say, "Oh, Neptune" or "Thank Neptune", as Neptune is the god that sea creatures worship.
Shrimp has been used on occasion as a euphemism for shit depending on the context.
In the 2003 Teenage Mutant Ninja Turtles animated series, the turtles would often use the word 'shell' where a more adult show would curse — "What the shell?" was most common, but they also had "Aw, shell"; "Holy shell!"; and "shell-for-brains." It's even in the Theme Song - It's a shell of a town!
In one episode in the "Fast Forward" season, Splinter sees that Raphael entered into a professional wrestling match. When confronted, Raph says, "Aww fish-sicles!"
In Danny Phantom, Mr. Lancer constantly uses book titles as expletives of shock. Ex.: "Moby Dick!", "Gulliver's Travels, I'm losing my mind - and my pants!", and "Lord of the Flies! They're slipping right through my hands!" "War of the Worlds, creature, get away from my youthful charges!" "Chicken Soup for the Soul!" "Hunt for Red October!", when the faculty's steak dinners are stolen, "Paradise Lost!" and, beautifully, when locked in a closet, "The Cask of Amontillado!"
The other bizarre expletive user, Vlad Masters. Unlike Mr. Lancer, however, Vlad uses food items such as "fudge buckets", "butter biscuits", "cheese logs", "butter brickle", etc. All of which practically mean Oh, Crap!.
The Angry Beavers make frequent use of the word "Spoot." In one episode, Scientist Number One actually shouts "Oh... expletive!" in shock.
Warmonga uses "frakkle", although she says it so casually it may be the equivalent of "darn" or "whoops" rather than anything harsher.
Mr. Barkin has several of these, including "cheese and crackers", from which it's not hard to deduce a prime suspect inspiration.
After Drakken complains about Jack Hench's prices, Shego offers to infiltrate his facilities and "find some 'free samples'". It takes Drakken a moment to realize that she means "sneak in and steal stuff".
There's a kind of variation in the 1970s British animated puppet show The Clangers. None of the title characters had any actual dialogue. They all "spoke" in echoing slide-whistle notes that had the cadence of English speech, so that it was often possible to work out what they were saying from the context of the story - for instance, "Whee-oo, woo-oo woo-oo" could be interpreted as "Hello, Tiny Clanger". On one occasion it was claimed that one of the character's whistles meant "Sod it! The bloody thing's stuck!" although this could never be proven. As a matter of fact, the offending phrase was used in the voice-box for the Clanger toys, and is "Oh sod it! The bloody thing's stuck again".
In a later version of the Care Bears franchise, Grumpy frequently uses "grumbly" expletives such as "Bumbling bittlebots!" and "Galloping gearbox!"
Tek Jansen, eponymous hero of the animated Show Within a Show on The Colbert Report, has a new curse every episode — usually space-themed puns ranging from "Venus Flytrap!" to "Space Mountain!" "Solar Plexus!" has been used repeatedly to the point where it's almost a Catch Phrase.
Fij Fij of Maryoku Yummy always says "Yappin' Yumblebum!" when something bad happens.
Plus there's Hawkgirl telling the Flash that he wouldn't have a good chance with Fire because she's "Brazillian".
One The Berenstain Bears cartoon has an episode about how cursing is wrong. The curse word? Furball. It's apparently treated as an ethnic slur in Bear Country. Despite being a largely homogenous society.
In the original book, it is never stated what the word is. It is implied, however, that it is the F-word (and I don't mean "furball"). The TV writers couldn't find a way to hide the word, chickened out, and went with the Unusual Euphemism.
An episode of Arthur, dealing with the same subject matter, did have the guts to use a bleep, as in actually showing that, yes, D.W. was actually swearing.
Animaniacs: In Goodfeathers shorts, pigeons use the word "Coo" in dirty ways, such as "Coo you" and "Coo off". In one episode where one of their girlfriends leaves them for another, Pesto says to the new boyfriend "have you been cooing my girl?"
Principal Skinner will use this on occasion, the most memorable being the exclamation "GM Chrysler!" Similarly, Mr Burns is fond of this: most apparent is '22 Short Films About Springfield' where he motivates a bee-stung and quickly-dying Smithers to continue powering their bicycle-built-for-two in completely accurate 19th century slang insults—calling him (among other things) a "stuporous funker." Wikipedia has a breakdown here.
In the Treehouse of Horror segment "Starship Poopers" upon hearing Maggie's distress call Kang exclaims "Holy flerking schnidt!"
In the PJ Sparkles pilot cartoon, after seeing that his evil plot has failed, the villain cries out "Oh, spit!"
Elisa in Gargoyles occasionally uses "Jalapeños!" as a general-purpose curse. Actually, Goliath and Broadway started that trend (Goliath first shouted "Jalapeño!" appropriately enough, the first time he ever ate a jalepeño), and Eliza picked it up due to her hanging with the Gargoyles.
Family Guy needed an Unusual Euphemism to get past the network censors, so they invented the word "Clemen". They noted that it would soon gain some obscene meaning and they wouldn't be allowed to use it again.
Not quite, it was a spoof of tabloid journalism. Tom Tucker announced it on the newscast as the hot new swearword, and that viewers would have to wait until after the commercial break to find out what it means.
They also had an episode where they sung the song "Shipoopi" from the 1957 musical The Music Man. People wondered if it was some kind of Unusual Euphemism even though the song itself explains that it means "The girl who's hard to get".
In Blue Harvest, they use Phantom Menace as a swear. (In geek culture, that actually is an offensive term.)
Used in the form of a series of increasingly ridiculous gestures made by Peter to try to imply to Death that he might get lucky with a girl.
In Metalocalypse, the band orders Ofdensen to use the term Hamburger Time for death to make it sound more pleasant. As a reminder, the band is a death metal band.
Katy from Katy Caterpillar is fond of exclaiming "Whippety Pow!" when she's excited.
An episode of Home Movies has a veterinarian refer to euthanasia as "making cotton candy."
An episode of The Backyardigans called "The Great Dolphin Race" has "Jumping jellyfish!" as an euphemism. It also has "Leaping lobsters!" and "Holy cowfish!".
Phineas and Ferb has the following exchange. The way the expression comes off is a bit confusing; it's implied that they think she's dancing and the song that they perform follows that logic. Though, at the exact moment the line is spoken, Candace is bending over with her hands on her butt and begins to shake her hips. It does look like dancing, especially when you consider that they are supposed to be breakdancers/rappers.
Candace:[screams] There's squirrels in my pants! Rapper: Wow! That girl's got some serious squirrels in her pants!
Lumpy Space Princess uses "lump" or "lumping" as a replacement for all swear words.
Several characters use "Glob" as a replacement for "God" in exclamations, but it later turns out that "Glob" is one of the four aspects of Grob Gob Glob Grod, a god-like entity widely worshipped in Ooo.
Megas XLR: The Glorft leader uses "I'll have your Jorblachs!" to chew his lackies out. Jorblachs most likely being the Glorft word for ass.
In "Bad Kiss", after a, well, bad kiss with Margaret during which he had bad breath and was pushed away, Mordecai uses a time machine in order to fix the ordeal. When he tries to convince past!Mordecai to remember about his breath this evening, past!Mordecai doesn't believe him and accuses him of "lip-blocking" him.
More recently, they've used "punching the Prime Minister" for urinating.
It apparently has some in-universe euphemisms such as "horseapples", "what in the hay", and "ponyfeathers", still these words are not commonly used. "Ponyfeathers" in particular appears to be a Curse of the Ancients. They also love to say "everypony", anypony, etc, even when referring to non-ponies like Spike and Zecora.
In "Putting Your Hoof Down", Fluttershy doesn't give a "flying feather".
The periphery demographic has a few as well. "Plot" came to mean a pony's rump (after the phrase "I watch it for the plot" went viral), implying that a "plothole" in a FIMfic is unrelated to its bad writing — in the term's usual sense. They also tend to use "buck" as a synonym for the F-word, leading to interesting implications for Applejack's use of "apple-bucking" to get the apples out of the trees.
In "Franklin and the Amazing Stupendous Circus Trick" from Franklin and Friends, Beaver uses "Oh, woodchips!" Additionally, in "Franklin Changes the Rules," also from Franklin and Friends, Rabbit bursts out with "Oh, carrot sticks!" after Beaver comes up with another burdensome rule for her pirate treasure game that nobody is really inclined to follow.
In The 7D the dwarves use the term "floom" in place of curse words.
The Venture Bros. gave us "Holy dammit Christmas!" Yes, "dammit" is technically an actual curse word, but its use in this case is still very weird, not to mention grammatically incorrect.
Parodied in Gravity Falls when Grunkle Stan drops a barrel on his foot and yells "HOT BELGIAN WAFFLES", but then remembers he's alone and he can "swear for real". "SON OF A-"
On Toot & Puddle, Puddle sometimes exclaims "Ah, thunderbugs!" when frustrated or upset.
Mr. Bumpy from Bump in the Night would sometimes use "bump" as an expletive, most notably saying "Bump this" whenever he was fed up with something.
On Dinosaur Train, "Zeppelin: Crater" has both "holy herbivores!" and "leapin' lambeosaurs!"
Kaeloo: The trope is parodied. In one episode, Mr. Cat offers to take Kaeloo on a dinner date and implies that they can have sex afterwards using a euphemism about "accountants" and "checks". Kaeloo, being naïve and innocent, believes that he's trying to bribe her.