Are they so named because they are unusually small, you ask? Heh, heh, heh...
Rats, As You Know, are probably the most formidable and tenacious mammals in existence, being blessed with swift feet, durable incisors, impressive cunning and intelligence, numbers, and an all-consuming sense of self-preservation. The only thing they lack, it seems, is physical size and powernote and longevity (the average rat is very lucky to make three years) but don't expect to see that brought up often. Then, given that final boon, they would surely transform from shadow-scurrying scavengers to feared, flesh-rending predators that would have leather-clad barbarians knocking their knees together.
Incidentally, if you don't find the regular-sized rats particularly worrisome already, then we hasten to point out that they are also world-class swimmers and can hold their breath more than long enough to reach the other end of the pipe leading to your toilet. (Thankfully, giant rats can rarely fit through the plumbing.)
Should you encounter these furry freaks, your best defense is to have a Mega Neko by your side. Subtrope of Animals Not To Scale. If in a video game, they're likely to be involved in a Rat Stomp. Occasionally, they're conscripted to provide Hamster Wheel Power for large machines or vehicles.
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Galaxy Angel had this alien hamster thing which they found in the ruins of a once great city.
Yu-Gi-Oh! has a monster outright called "Giant Rat". Then there are Nimble Momonga, which is just a very large flying squirrel, and a bunch of other more pathetic low-level monsters, like Beaver Warrior, which looks more like a rat than a beaver, but is a rodent either way. Some, like the Giant Rat, Nimble Momonga, and Super Nimble Mega Hamster, can be useful in swarm tactics.
Aside from the stuff mentioned in Video Games, one Pokémon episode had a giant robot Pikachu.
Neil Gaiman's gag biography in The Sandman: The Season of Mistsdenies that he was found outside a London sewer unable to say anything more than "Powerful big rats, gentlemen". And then goes on to deny that he had a vestigial tail, played a part in the obviously fictional negotiations between Londons Above and Below, or that there were any tooth marks on the bones.
In the Judge Dredd universe they have these. After examining the meat from one they decide to farm them in place of the regular rats they used to farm.
Beasts Of Burden has a Rat king leading the sewer rats, and his general is a rat larger than the two cats who fight him.
There's a Donald Duck comic where Don and his cousin Fethry team up to fight giant rat ghosts. Unusually big rodents shouldn't be a foreign concept to Donald, considering whom he used to co-star with early on his animated career...
Prince Raffendorf, from Larry Elmore's Snarfquest comic strip, was a human prince before being turned into a giant humanoid rat by an evil wizard.
In Bone, the Giant Rat Creatures, despite that they are drawn without snouts and that they cut off their rat tails as a cultural ritual.
One supporting character from The Elementals was a wererat who had a crush on Fathom.
Several Kia car commercials feature giant hamsters, sometimes complete with giant hamster wheels.
In an ad for Doritos, a man puts a piece of an extra-cheesy dorito on a mousetrap, then sits down to eat some more. A giant mouse (well, a man in a mouse suit) bursts out of the wall and tackles him, presumably not being satisfied with the tiny tip of one chip.
Orkin's series of Giant Creepy Crawly extermination-service ads now includes one in which a family comes home from a trip to find scruffy human-sized rats hanging out in their living room.
There's a commercial for extra-durable work pants which demonstrates their toughness with a giant cartoon beaver, which loses its teeth trying to bite through a pair.
Named after the R.O.U.S. from The Princess Bride. (When Wesley tells Buttercup that he thinks they don't exist - see the quote above - he's lying to convince her to calm down. He had seen the monsters clearly a few minutes earlier.)
The movie adaptation of H. G. Wells' The Food of the Gods features giant rats besieging some people in a cabin. Or rather, it features normal rats romping around a miniature set, and a few prop rat-heads that make "The Talons of Weng-Chiang" look like Aliens by comparison.
The film Altered Species has a giant lab rat released by protesters who thanks them by trying to eat them all.
In one of the Eddie Murphy The Nutty Professor movies, the Professor accidentally creates a giant hamster.
Of Unknown Origin is a surprisingly good movie about a New Yorker who's terrorized by one of these. It's not a mutant. It's not an alien or magical. It's just a big, mean, nasty, and EXTREMELY determined Rattus norvegicus, which figures his apartment is its territory.
In the remake of Willard, the role of Ben was played by a Gambian pouched rat, making him far bigger than the rest of Willard's colony.
Let us not forget Rats: Night of Terror set in a post-apocalyptic future where the rescuing men in the radiation suits take off their gas masks to reveal that...well I think you know that they ain't gonna be human!
The bizarre movie Black Moon featured a truly massive, cat-sized rat (probably an African Pouched Rat) that could talk.
Italian movie Quella Villa in Fondo al Parco (aka The Ratman) has a genetically engineered rat-human hybrid wreaking havoc.
The formerly human rat monster in Bottom Feeder.
Mulberry Street has a virus break out in Manhattan, one that causes people to mutate into homicidial rat creatures.
The title dollhouse in Amityville Dollhouse transforms a regular mouse into a giant one. It dies when the house is tipped over.
Technically, of course, shrews aren't rodents at all. Close enough for this trope's purposes, though.
Although it's normal-sized, the squirrel in the second Night at the Museum plays this trope straight from inch-tall Octavius's POV.
1984 by George Orwell features enormous rats, apparently capable of chewing straight through a man's head. The Party uses them as a torture device on Winston in Room 101. And Winston is terrified of rats.
The Stephen King short story Graveyard Shift has a lot of rats of usual size...until you go down to the sub-basement where there are not only rodents of unusual size but they're mutated as well, the "queen" of which is big enough to eat a man(appears in the collection Night Shift).
Reepicheep (and the other Talking Mice) from the Narnia series, who is described to be two feet tall.
And he knows no fear.
The squirrels too. For some reason, all the Talking Animals in Narnia that would be smallest in our world are slightly larger there, while the biggest ones (like elephants) are slightly smaller. This is Lampshaded in The Magician's Nephew.
Given that even Peter could enter their home with ease, Mr. and Mrs. Beaver must've been quite a bit bigger than normal as well.
The Goosebumps book Monster Blood II had a 10-foot tall hamster in it.
In The Resistance, the forty-seventh book in the Animorphs series, the team morphs beavers to create a dam as part of a last-ditch battle strategy against an impending Yeerk invasion force.
The speculative pop-biology book After Man: A Zoology of the Future posits a world 50 million years hence where carnivores have gone extinct and rodents have evolved to fill the niches left by felines, canines, bears, weasels, wolverines, and so on.
And Fred Saberhagen picked up that dropped thread in The Holmes/Dracula File, which reveals why the world was not prepared. It's not its size that makes it dangerous, but the virulent plague its fleas carry.
In the Doctor Who/Holmes crossover All-Consuming Fire, it's an alien being.
And in Rick Boyer's The Giant Rat of Sumatra, it's a tapir.
The last verse of Genesis's All in a Mouse's Night has a cat getting beaten up with one blow by a giant mouse.
The rats from The Nutcracker. Although, technically they were no bigger than the Nutcracker...
Ratmen (and a genius ratgirl) play a part in Glen Cook's Garrett P.I. series. It seems the wizards of a few generations back had quite a fad for the creation of new sentient lifeforms, and they shared real-world scientists' preference for using rats as research subjects.
Like other mammals, rodents were well-represented in the Spellsinger novels, although rats and mice were treated like second-class citizens, forced to wash floors and so on.
It seems to be a common subversion that if rats aren't the monsters or the bad guys, they're the Butt Monkey race.
The rats in Mrs. Frisby and the Rats of NIMH were Rodents Of Unusual Smarts.
Brutus, while certainly not giant, was described as being rather large as rats go.
It was stated that the drugs they were given increased their size...it was part of why the normal rats feared them.
The only non-mutant human creatures seen in the future in Mindwarp are giant rodents the size of a capybara. It is hinted they are descended from rats.
The children's novels The Castle in the Attic and The Battle for the Castle, by Elizabeth Winthrop, are about a kid with a magical miniature castle. Through use of a magic token, he can become small enough to enter the castle—and the entire medieval world beyond it. Battle features a battle with giant rats, which makes sense if you think about it, since the rats in the attic don't have magic tokens...
Lampshaded in the Russian play "The General Inspector" by Gogol. One character dreams of two "Rodents of Unusual Size" the night before becoming a letter that said inspector is secretly coming to his town - and since he is an Obstructive Bureaucrat, it's a very bad thing indeed.
Redwall: In The Long Patrol, Big Bad Damug Warfang is a Greatrat, described as twice the size of a normal rat.
While the titular rodents of Paul Zindel's Rats are generally of normal size, the book also features the Rat King (no, not that one), which is described as being even bigger than a capybara.
In The Borribles, the race of Rumbles are described as rat-like, and are the size of human children.
Parrish Plessis has canrats, rat-dog hybrids that are both vicious and intelligent. One of them, the Big One, is the size of a doberman.
H. G. Wells' novel The Food of the Gods features giant rats, about the size of wolves, as part of the mutated ecology that the titular food's unleashed. Unfortunately for humans, the rats also have the carnivorous temperament of wolves and quickly become the dominant pack hunters in the hot zones.
The Changelings from The Amazing Maurice and His Educated Rodents tend to be larger than keekees (normal rats), presumably because their intelligence lets them keep themselves better-fed and healthier. Some of the normal rats bred for the fighting-pit by the ratcatchers are also larger than average.
The Lemming-Men of Yull from Toby Frost's Space Captain Smith books have armed and industrialized themselves, but still retain their love of jumping off cliffs.
The Doormouse is a once-human businessman in the Nightside, who had himself changed into a giant bipedal mouse because he likes being cute and fuzzy. Not a dormouse; his name came about because he's in the business of renting out use of his Cool Gate collection.
Alan Dean Foster pulled the same pun in his Spellsinger series, although the doormouse (majordomo in a brothel) was in fact a dormouse. (Spellsinger rodents are much larger than their Earth equivalents, even more so than in Narnia.)
The average rat in Domina is about the size of a small dog. That's what happens when you let mad scientists play with a Bio-Augmentation device.
The Rh/attes are an aptly-named alien race from Chess With A Dragon.
In Myth-Taken Identity, the mall-rats are about the size of spaniels, while their boss Rattila is twice their size. In the previous book, Myth Alliances, a big rodent-like beast is seen pulling a cart on one of the visited dimensions.
In Fort Freak, a Wild Cards novel, Vincent "Ratboy" Marinelli is an effective member of the Department of Internal Affairs, aka the Rat Squad. He is a "Joker" w/ the shape of a giant (4-foot) brown rat. For a little more, see another other wiki: http://wildcards.wikia.com/wiki/Ratboy
In Simon Hawke's The Wizard of Rue Morgue, Wyrdrune is ambushed by a rat the size of an elephant in the sewers under Paris.
Technically true in Wayne Barlowe's Expedition, in that environmental degradation on Earth has become so severe that the (ordinary) Norway rat is the largest wild animal left on the planet.
In Death Warmed Over, the zombie foreman of a garbage dump has giant rat pets that act like dogs.
On Sabrina the Teenage Witch, we hear the Quizmaster on the phone with an exterminator complaining that the rat in his apartment in the Other Realm is making long-distance calls on his phone and listening to his CDs without putting them back. (Whether he pays rent or not is not the point.) They are apparently unsuccessful since we meet the ROUS in another episode when he introduces himself as the Quizmaster's "roommate."
SPG, the pet hamster of Vyvyan on The Young Ones, was usually played by a puppet the size of a guinea pig rather than the size of a real hamster. In one episode, he scarfed down an entire potfull of lentils and swelled up to volleyball size.
In the Night Gallery story "Nature of the Enemy", astronauts find a large metal apparatus on the Moon. Just as the staff at CapCom have realized it looks like a mousetrap, the landing party's transmission cuts out, then resumes to show an empty space helmet being inspected by a building-sized rat.
Green Day had an album cover where a gigantic hamster was attacking society.
There's a New York punk band named Rats of Unusual Size.
The World War One song "The Quartermaster's Store" mentions "rats, rats, as big as bloody cats" (see also Real Life).
David Bowie's "Future Legend", the first track of Diamond Dogs:
Fleas the size of rats sucked on rats the size of cats.
There's a Japanese legend called "The Boy Who Drew Cats", in which a city is plagued by a giant demonic rat. Which is eventually killed by the drawings of cats by the boy in the title. It's magical, okay?
Also from Japan, Tesso (Iron Rat), a former human noble with Jerk Ass, ungrateful parents who was cursed by a monk. He's a man-rat hybrid that raids the temples and the houses with a horde of smaller rats and devour everything in his path.
This is an occasionalgag in Garfield; Garfield will be pretending to mouse, or maybe teasing a mouse and the mousewill belargerthannormal. Then there's the Training Mouse from the arc where Garfield got locked out and found his way back to where Mama Leoni's used to be. And then there's the subverted one where it only *looked* like a really tall mouse, the shot from behind the mouse hole shows it was three mice standing on each other's shoulders.
The infamous "Moon Hoax", a series of articles in the 19th century New York Sun, included fanciful accounts of giant civilized beavers living on the moon.
The underhive in Necromunda (part of the Warhammer 40,000 setting) is infested with these. And not just any giant rats - more intelligent, mutated giant rats. Some are spiky, some have two heads, but they all are happy to eat lone humans if they think they can get away with it.
Of course, humans are more than happy to return the favor. While the uppermost Hive Dwellers might feast on food exported from agri-worlds, the average Underhiver has a distinctly less pleasant variety of foodstuffs to choose between. The most "normal" foods are fungi and edible slime-molds. "Meat" in the Underhive generally comes from rats... or snakes... or spiders... or, really, anything made of meat that fails to climb out of the pot. The nastier folks are even willing to add their fellow man to the list. To say nothing of such delights as "Wild Snake", a popular booze made from a certain species of giant snake.
The Underhive also hosts a faction known as the Ratskin Renegades, whose theme is based on First Nations plains tribes, with ROUS filling in for bison and horses.
The Hrud of the same universe , sometimes called "Space Skaven", might also qualify, at least before their appearance was Retconned to some sort of squiddish thing.
"Dire rats" are a common low-level monster in Dungeons & Dragons that often show up in caves and city sewers. They're scavengers who won't hesitate to attack live prey if they think they can get away with it and carry disease more often than not. There are also wererats, humanoids cursed to change into dire rats much like werewolves.
You're forgetting rat swarms, skeletal rat swarms, corpse rat swarms (Zombies!!!), spectral swarms (incorporeal undead that typically result from careless fireball-flinging adventurers inflicting large amounts of collateral damage on the local rat population), cranium rat swarms (psychic rats!!!), moonrats (rats that become more or less intelligent depending on the phases of the moon), and the Tamer of Beasts prestige class from the book Masters of the Wild, who is depicted in the artwork as controlling a massive army of rats.
And there's the Rylkar from Version 3.5's Monster Manual V. They're basically a nest of giant, evil rats who are connected via a hive mind to their harridan, the huge, disease and corruption spreading, blind matriarch of the nest.
The Spelljammer supplement for 2nd edition D&D introduces the infamous Giant Space Hamsters, domesticated and bred by the Gadgeteer Genius Tinker Gnomes of Krynn, and coming in a wide variety of breeds including the "Miniature Giant Space Hamster", which is identical to an ordinary hamster.
Previous editions of D&D also included giant beavers, giant porcupines, and — I kid you not — giant carnivorous flying squirrels.
One highly popular adventure from the early days of Dungeon magazine shrank the PCs down to the size of gaming miniatures, making ordinary rats appear enormous by comparison. Other humanoids, who'd previously fallen victim to the same magic, used rats as steeds.
Magic: The Gathering has 'Rat' as a creature type, from the classic Plague Rats that only the four-of-a-card deck construction limit really keeps from growing arbitrarily dangerous to the Kamigawa block's nezumi (rat-people complete with their own warriors, rogues, shamans and ninja).
Relentless Rats was designed and printed to allow people to enjoy plague rats without the four-of-rule, explicitly stating that it ignores it. Also is much better.
The original art of Giant growth featured a giant rat. Now it's a bear.
There is also a changing breed in the World of Darkness called the Ratkin, that are sometimes born as humans (that have the ratkin genetics) and contract a disease that, should they overcome it, turns them into wererats. They were given the charge by Gaia to help control the human population by eating their food and spreading disease.
These Ratkin can take a talent to be able to transform into a giant rat that can stand approx. 4 ft. at the shoulders.
In Shadowrun, devil rats are Awakened rodents the size of a border collie. They're nasty, vicious, disease-carrying, and (for some reason) bald all over.
The New World of Darkness has Beshilu, one of the two iconic races of the bizarre half-spirit Hosts (the other being the Azlu). Like their cousins, they start off looking like normal rats, but quickly gain size and sentience as they eat real rats and lesser Beshilu. They then gradually gain the ability to control human corpses and eventually become humanoid, where they become far more social then other Hosts, froming tribal societies. That wouldn't be so bad-they don't prey on humans all that much-except that they also are driven by instinct to gnaw holes in the barrier between the Spirit World and the human one, which, given where they live (ie, where normal rats live), quickly becomes a haven for disease-spirits, who of course, exist to spread disease. And like other hosts, killing them simply causes a large one to split into a swarm of rats, with one of the component rats containing his soul-which, if left alone, will eventually grow back to full size and power.
Pathfinder features the usual assortment of giant rats featured in most tabletops, with a welcome addition; the friendly, gentle capybara(see Real Life, below), also called the donkey rat, is available for use as a familiar by spellcasting classes.
Pokémon has quite a few of these, often Com Mons. Examples include Rattata, Pachirisu, and of course, the Pikachu family.
Super Smash Bros. has very scary implications if it weren't for the fact that the characters are trophies. Imagine, if you will, dropping a life-sized Pikachu on Pop Star or Hocotate. A malicious Pikachu (or however many more) in either of those settings would be a Cthulhuesque horror upon the populations of those worlds, as it fits ALL of the ROUS criteria without the use of Super 'Shrooms.
Somewhat justified because they are usually cute, big eyed and not very menacing. Not even those with teeth bared are all that terrifying.
Raticate is slightly more intimidating than some of the others. And extremely annoying if you're up against one with that much hated Hyper Fang move.
Fallout 3 has a robot you can activate in the Old Olney Tunnels that mentions something during its startup sequence about an infestation of rodents of unusual size. It is usually torn to pieces soon after by the 10 foot tall Deathclaws infesting the tunnels.
There's also a woman in one of the cities that keeps pet rats. Her rival hated them, and so put out meat to lure them, which had the effect of attracting mountain lions which came and killed a few of the rats. The lions were surprisingly weak in battle compared to the ones you usually fight in the wild.
That's because they were starved.
In Skyrim they're called skeevers. They're so big that people lay down bear traps to catch them. One crazy guy underground tried to create an army of them.
At least one NPC notes that the skeevers used to be smaller.
Twitch, a champion in League of Legends, was a sewer rat who gained sentience and bipedal form from magical runoff. Lonely and a bit maniacal, his goal is to duplicate the phenomenon and create a race of sentient rats to rule over.
In the 1990s PC fantasy kingdom sim Majesty, giant rats were generally the first monsters to show up in your kingdom.
And rat-men were another common annoyance, though they were at least one of the few enemies your city guard could handle competently.
Puzzle Quest: Challenge Of The Warlords has giant rats, scorpions, bats, and wasps. The first two can be captured and ridden, granting the player a different stat bonus and additional spell, depending on which one you choose.
Suikoden II had a giant, mutated Sewer Rat for a boss. Which could attack twice per turn, and hit all of your party with each attack for a lot of damage. Goddamnrats.
Final Fantasy III had one too. It is unusual, in that it is a normal-sized rat, but your party has to use the Mini condition to reach the Plot Coupon it's guarding. Since the Mini status effect cuts Defense and Attack to 1, you're basically forced to go at it with a party of Squishy Wizards. Best bet is to change your physical fighters into Red Mages for the duration of the dungeon, since you don't have the advanced spells they're locked out of at this point of the game.
Tales of Symphonia has a similar situation where the party gets shrunk down in a sewer and meets the same itty bitty mice which they can encounter as GIANT KICKBOXING MICE.
Shining Force 2 has rat enemies of both varieties as the above examples. On the first battle on the field, the party encounters Huge Rats. Later in the game, the party gets shrunk down as part of the storyline and faces normal-sized rats. Your stats don't suffer the debilitating effects of Mini like in FFIII, but those normal rats are still hella strong. And led by a super-rat named Willard.
Not to mention Slade the rat thief who started the entire mess in the game by stealing the Jewels of Light and Evil. Including him in the fight with Willard causes instant Furry Confusion because Slade is anthropomorphic.
In the "Down the Tubes" and "Tube Race" levels of Earthworm Jim, the only way to get through several corridors full of tiny bruisers who will slam you around and throw you back where you came from is to ride a giant, foe-eating hamster ("Whoooooooa Nelliiiiiiiie!").
Mouser from Super Mario Bros 2 is a gigantic bomb throwing killer mouse boss. Who has probably the most ironic kind of name ever for such a creature (considering the word 'mouser' means 'cat which catches mice').
Everquest II's Ratonga are prime examples of what happens when you give a ROUS opposable thumbs and knives. An entire race of automatically Evil aligned ROUSes with a penchant for being thieves and assassins. Throw in Roekillik, their Minime counterpart race, and it seems someone at Sony rather likes this trope. There are also regular giant rats (also found in the first Everquest).
Every roleplaying game Spiderweb Software has ever made, with the exception of the original Geneforge, has giant rats in it. They're usually the very first enemies you fight before you go on any quests.
Castle of the Winds has Giant Rat, but it's pitifully weak. No, it's the ants that new characters should watch out for.
AdventureQuest has BURPS, which stands for "Big Ugly Rat Pests". They're Exactly What It Says on the Tin. And every other year there's a war fighting nothing BUT those guys. They also qualify as Goddamned Bats because they're a pain in the ass to hit, where as the bigger ones deal quite a bit of damage. At higher levels you'll still be fighting the guys, often in groups.
It gets better—one of their variants is actually called the ROUS.
Parasite Eve had mutated rodents that tried to kill you. And giant squirrels, too.
Played for laughs at the start of The Bard's Tale, where the eponymous Bard goes into the basement of a tavern to kill a rat for the hostess. After some patronising dialogue from the narrator, a giant rat emerges from the darkness, and breathes fire on the Bard, forcing him to retreat back above ground. Turns out it was all just a prank, which the drunken patrons got a good laugh out of.
The DuckTales video game for the NES had that giant rat boss guarding the Green Cheese treasure in The Moon stage.
Played with in Baldur's Gate, where Minsc's pet rodent Boo is an unusually small "miniature giant space hamster". "Go for the eyes, Boo!"
Mirror's Edge breaks realism to include one, if you fire at a certain sign with a sniper rifle.
Resident Evil Outbreak File #2 features those rats that spread the T-Virus attacking one of HUNK's men after he'd been felled by Birkin. There was also artwork showing muntant rats that didn't make the game.
Tomb Raider has rats that are at least the size of a medium-sized dog once you reach the "Cistern" stage in the Roman section of the game, given how other animals were generally a believable size up to this point, it was noticeable.
Metro 2033 and Metro Last Light feature a number of seemingly rodent-derived monsters among their menagerie of radioactive mutants. Lurkers are small, fast creatures that resemble oversized naked mole rats, while the larger and more vicious nosalises look more like they were originally moles or shrews of some sort. Both mutants are incredibly hostile and have an acute taste for human flesh, with the nosalises in particular having developed massive, fanged maws that can easily rip people to shreds. Watchers/Watchmen are harder to place; they have some rat-like features, like their hairless tails and ability to stand on their hind legs, but overall seem more like wolves instead (hunting in packs, howling, etc...).
Giant mutant rodents are the signature creation of Narbonic's Helen Narbon. Of course, Helen being a young girl at heart, they happen to be giant mutant gerbils.
The trope name is evoked in the title of an 8-Bit Theaterepisode which discusses dire rats... but the resulting rodent isn't one, just an Ax-Crazy dwarf disguised to chase for it.
Even so, sixty centimeters is pretty sizeable for a rat; especially since, during the approximate time period Girl Genius takes place in (assuming sometime during the 1700s, given a couple hints), the most prevalent variety would be the black rat, as the Norway rat was at that point only getting started driving the black rat to warmer areas. Norway rats are noticeably bigger than black rats, and even for them, sixty centimeters (about two feet) tip-to-tail is pretty big.
Secretary, the second arc of Nature of Nature's Art, plays with this trope. All of the important characters are rodents in this arc, and some are bigger than others. In the end, though, the only one who plays the trope straight is the advanced class teacher - he makes degus (SV and NT) and chinchillas (SV's teacher) look small, and early in the arc, a mouse called NT "huge".
Hamstard, the Bastard Hamster mascot of Erfworld's in-character blog, qualifies by virtue of being incredibly fat. Really, Parson should've gotten the little blob an exercise wheel before being swept off to another dimension...
In the Blade of Toshubi we have Toshubi, a human-sized anthropomorphic mouse from a village of human-sized anthropomorphic mice.
In Yamara, Tim the paladin is turned into a vampire, but messes up his first attempt to turn into a bat, becoming a giant flying squirrel instead.
In Weregeek, when the GM mentions rodents of unusual size attacking the party, they protest that given their statistics of rodent encounters, that size is the usual one.
Actually older than Web Original, as it goes back to Usenet, the Internet Oracle has as his arch enemy Woodchucks. The reason is the infamous Woodchuck question he is constantly asked, "How much wood could a woodchuck chuck if a woodchuck could chuck wood?". Some of his enemy woodchucks were rather large. After The Princess Bride came out, they were given the official name of "R.O.U.S.".
Squirrels are one of the eight sentient (i.e., humanoid) species in Tasakeru. They're the only rodent species among sentientkind; the others are canines (wolves and foxes), mustelids (badgers, ferrets and skunks), procyonids (raccoons), and lepines (rabbits).
More than one cartoon from Looney Tunes involved Sylvester thinking he's encountered this trope, but it's really a young kangaroo that keeps swapping places with the mouse he's after and smacking him around.
And of course Jerry pulled the same trick on Tom, with a baby elephant.
In the Looney TunesThe Great Piggy Bank Robbery, Duck Twacy follows footprints to a teeny-tiny mousehole and deduces the criminal Mouse Man is inside. He shouts "Come on out, ya rat!" and a HUGE snarling rat-in-a-suit pops out looming over Twacy, who whimpers "*gulp*...go...back...inside."
In Back at the Barnyard when everyone got a clone, said clones were all mini-sized.. except Pip's, who went the other way.
Alvin and the Chipmunks in their animated incarnations. They're chipmunks and are never implied to be anything but chipmunks, but they're generally about four feet tall. The newer CG-animated remake has them at actual chipmunk size.
Alcazar's friend Rat Man and his girlfriend, from Futurama.
The Tex Avery classic King Size Canary has a cat trying to make a decent meal out of a puny canary by feeding it fast-growth plant food. It works alarmingly well, and soon the cat, the bird, a mouse, and a bulldog are all taking swigs of the stuff, jockeying for size supremacy. The cartoon ends with the cat and mouse waving goodbye to us, standing on a relative beach ball sized planet Earth.
One episode of Cyberchase involved The Hacker using a giant hamster called a hamborg (which for some reason resembled a capybara) as part of his evil plan.
In the Wile E Coyote And The Roadrunner cartoon "Chariots of Fur", Wile E. tries to catch the Roadrunner with a giant mousetrap. Instead, he snags a giant mouse, who then turns the tables on him.
In the when-nightmares-attack episode of G.I. Joe, Lowlight is revealed to have recurrent bad dreams about hybrid rat/car horrors stalking him at night in his father's junkyard.
Ben 10: Omniverse: Rook picked up his impressive fighting skills defending his planet's wheat silos from the local rodents, which, according to him, "Run fairly large."
On an early episode of The Simpsons, Bart is convinced by Sherri and Terri that the Mayflower left England to escape from giant rats.
The capybara, a South American rodent the size of a small pig.
Trench rats in World War One were often reported to grow to the size of house cats, because of their constant engorging on the corpses in No Man's Land and the soldiers' food. And when feasting on the corpses, these bloated rats ate out the corpses' eyes first. This site has a little article about the trench rats in WW 1. Imagine sleeping with overgrown, bloated rats the size of a house cat running across your face. Nightmare fuel, no?
There is an entire episode of the t.v. show Monster Quest that deals with sightings of cat or even dog-sized rats in major U.S. cities like New York, including a homeless man who reported a 3-foot giant in an abandoned subway tunnel.
In New Orleans, Coypu (once farmed for their fur) have established breeding populations along the city's levees and waterways, where they are sometimes mistaken for Rats Of Unusual Size. These semiaquatic mammals are actually from a different family of rodent than rats, and are supposed to be the size of tomcats.
If you ever see a "giant killer rat" in a sideshow, it's probably a coypu. (They used to use capybaras, but those are incredibly high-maintenence.)
The Gambian giant pouched rat can grow to over 2' long, and is one of the largest rodents to be formally classified as "rats". They've been trained to sniff out land mines in Africa, which kinda subverts this trope's "feared, flesh-rending predator" aspect.
The tragically critically-endangered Cloud Rats of the Philippines. As cute as living stuff toys!
And according to this article, the largest known rat of unusual size has recently been discovered in a crater in Mt. Bosavi. And it's completely docile, too.
Prehistoric rodents could get absolutely gigantic: Neochoerus pinckneyi is a Capybara 40% larger than its modern cousin (200-250lbs); Casteroidies, a beaver the size of a VW Bug (8ft long, 200+ lbs); Phoberomys pattersoni is one of largest of all known rodents, growing to almost 10ft in length with an additional 4ft of tail, weighing up to 1,500lbs; The largest goes to Josephoartigasia monesi which did hit 10ft long, stood 5ft tall at the shoulder and weighed over a ton (it's incisor teeth were a foot in length). A lot of these giant South American rodents are basically even bigger capybara.
Tehran, Iran, currently suffers from a severe rat infestation, with an estimated 25 million black and Norway rats occupying a city with about half that many people. Some of the rodents reportedly grow to at least 16 inches long and 5 pounds in weight, suggesting these might actually be of a larger species that's begun to displace the others.
Ironically, millions of medieval people probably owe their lives to the arrival of Rodents of Unusual Size, namely the spread of the larger Norway rat into territories previously occupied by the black rat. Of the two major rat species that infest human communities, the smaller black rat is far more prone to transmit bubonic plague to humans via its fleas, so the Norway rat's displacement of its weaker cousins throughout much of the world helped to reduce the frequency and severity of plague outbreaks.
Breeders of fancy rats have begun developing varieties for the pet trade, including a "goliath" strain intended to be larger than usual. As such breeding programs are just getting started, "goliath" rats will need many more generations before they're even double the size of normal fancy rats.