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The rodents of unusual size in question are not implausibly large from a biological standpoint, being roughly the same weight as an American Beaver, but much more adapted for land living and with an omnivorous diet. But the problem stems from where they live.

These rats are thriving in a dense urban environment, and I am unsure if an urban environment can support a population of larger animals. My initial idea is that there is lots of human and animal waste simply lying about since it’s a disgusting city. But I’m not sure if it’s a plausible explanation or if there’s something I’m missing that would prevent a larger animal from living off trash and hiding in abandoned alleyways.

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    $\begingroup$ On eo fthe advantages rats have for liing hin humanbuildings and ocmmunities is the abilty to squeeze thorugh tight s gaps and live inside hollow walls. Giant rats the size of beavers would lose that ability. The biggest rat I ever saw was about the size of my boot or a possum. But its body seemed for rat like than possum like, I don't know what species of rat it was. $\endgroup$
    – M. A. Golding
    Commented Oct 21, 2022 at 4:11
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    $\begingroup$ You are describing raccoons, a very successful urbanized species. They aren't typically found deep in cities, but given another 100 years, seems plausible. $\endgroup$
    – user458
    Commented Oct 21, 2022 at 15:24

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Yes, there is a precedent for large animals inhabiting urban areas.

The typical mass of a adult American beaver is 20 kg. Feral dogs living in urban areas are known to exceed this at times. Although 20 kg is on the large side for dogs, some dogs are much heavier. Note that the article mentioned below specifically mentions that they may be any breed.

Are feral dogs present in cities? Yes, they are, and present a problem.

Feral dogs, as well as feral cats, are rapidly becoming a very serious problem in most if not all of the larger metropolitan areas throughout the world. Unfortunately these feral dogs, many which have been abandoned by uncaring owners, have not only survived their life on the streets but have also gone on to reproduce. This has further increased the numbers and has lead to some very real health and genetic concerns within these numbers. Feral dogs can be from any breed and most, especially if born as feral puppies, are going to be mixed breed, perhaps mixed breed for several generations.

Now, humans keeps abandoning pets which provides a continuous feedstock for wild animals, but as the article explains, such wild populations do breed. It is obvious that cities have dog-catchers and impound lots because of the population of animals that run loose.

There is no reason to doubt that ROUS could survive in an urban setting. Besides, have you seen how big the existing rats are in NYC? NYC Rat

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  • $\begingroup$ I was going to mention the NYC rats. I've only ever been in the city once and got a little freaked out when I saw one scurry down some subway tracks while I was waiting for the train. Apparently the various tunnels in Manhattan have legit infestation problems. $\endgroup$
    – hszmv
    Commented Oct 21, 2022 at 13:32
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    $\begingroup$ And "dogs" really highlights what I think is an important point: what makes large rats have a hard time in cities is that people see them as disease carrying vermin and actively seek to eradicate them. All the OP really needs is a culture that sees rats as "cute and friendly" (presumably with no historical connection to plagues) and they'd probably be viewed similar to dogs. Or raccoons (which are seen as pests, but basically mild, cute, adorable trash pandas). Oh, see also: monkeys. Some cities are absolutely infested with monkeys. Or cats. $\endgroup$
    – JamieB
    Commented Oct 21, 2022 at 15:18
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Larger animals can live in urban environments, like for example foxes do. However if you increase the size of the animal you have to reduce their total number in the environment: one thing is feeding rats, one thing is feeding something larger.

With the same nutrient base, the larger the body, the smaller the total count of the animal.

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  • $\begingroup$ This is a fair point, and these rats are dense but not as dense as real rats (I was thinking packs of 5 or so) $\endgroup$
    – user71781
    Commented Oct 21, 2022 at 4:09
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Here are your Giant Rats

enter image description here

What you describe is essentially a raccoon. A smart, nonaquatic, opportunistic omnivore two feet long that loves to eat garbage. They do not live in my part of the world, but I have seen in cartoons that American cities are full of raccoons. This is much to the chagrin of the American people. American garbage tastes delicious you see. And every kilo eaten by raccoons is a kilo stolen from the hard working citizens.

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  • $\begingroup$ And, as seen in the NASA documentary Guardians of the Galaxy, raccoons can learn to pilot spacecraft... WIkipedia says: "Since the 1950s, raccoons have been present in metropolitan areas like Washington, DC, Chicago, and Toronto. Since the 1960s, Kassel (in Germanyt) has hosted Europe's first and densest population in a large urban area, with about 50 to 150 animals per square kilometer (130 to 390 animals per square mile)," $\endgroup$
    – AlexP
    Commented Oct 21, 2022 at 17:34
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I don't know if you're writing fantasy or science fiction, but much might depend on ...

  1. whether the rats were sapient
  2. whether the rats preyed on small children
  3. whether humans used the big rats to exterminate the smaller ones and other vermin
  4. whether humans used the rats as pets
  5. whether humans used the rats as permanent surrogate babies or toddlers
  6. whether rats could convincingly disguise themselves as cats, dogs, or people
  7. whether the rats were social and could build gargantuan nests or systems of burrows the size of houses
  8. whether the rats were social and guided by a sapient queen
  9. whether certain organizations wanted to stop the efforts of other groups to exterminate the rascals
  10. other factors.
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