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A wererat is a therianthropic creature in the shape of a rat. The term is a neologism coined in analogy to werewolf, used in the fantasy or horror genre since the 1970s. The concept has since become common in role playing games and fantasy fiction inspired by them. Were-rats are commonly portrayed as sewer-dwelling scavengers and opportunistic thieves.
In the Dungeons & Dragons fantasy role-playing game, the wererat is one of several different types of animalistic forms. They are classified as "lycanthropes", despite the fact that the term reflects a lupine, or wolven, form. In the Oriental Adventures supplemental sourcebook, they appear as the Japanese mythological creatures, the Nezumi; these "ratlings" as they are often called by humans, are a race of bipedal ratlike humanoids. They are also found in various games inspired by or based on Dungeons & Dragons such as Legend of the Five Rings, NetHack and Neverwinter Nights.
In the Palladium Fantasy Role-Playing Game, wererats are one of the species of werebeasts that dwell in the Darkest Heart, a magically corrupted forest found in the southern region of the Land of the Damned. Wererats, along with many other werebeasts, are relatively unheard of in the Palladium world outside of the mysterious Land of the Damned.
In Warhammer Fantasy, the Skaven (1986) are a subterranean rat-like race. While not wererats, per se, they are very close in appearance. They are divided into many clans, each seeking power. These include Clan Moulder, mad users of alchemy and mutation magic, Clan Pestilens, the plagued rats, Clan Eshin, a secretive assassin clan, and Clan Skryre, insane magic engineers. All clans use the mysterious Warpstone to further their causes in their own backstabbing ways. The Skaven are treacherous, backstabbing and ever mistrustful towards each other; it has been hinted that this infighting is the only reason they have not yet taken over the world.
In Arcanum: Of Steamworks and Magick Obscura, wererats are enemy monsters that can be found in the wild, in the sewers and in other places.
The popularity of wererats in modern fantasy may have been pioneered by the Fafhrd and Grey Mouser novel The Swords of Lankhmar (1968) by Fritz Leiber. While there is no shapeshifting per se, there are normal-sized rats with human intelligence, humans who change size to pass between human and rat society, humans who are half-rat, and a rat city below the human one.
In the American Manga Gold Digger (1991) wererats were one of several lycanthropic races created by an enchanter named Iceron (later revealed to be the Wererat elder Gothwrain). Five characters important to several recent events are wererats. Sherisha, the near immortal leader of the wererat clans, Gothwrain her thrall, servant and apparent enemy, and also three young warriors/assassins collectively known by fans as the "Mall Rats" : Lydia McKraken, Romeo Ellis and Moisha Rich.
Splinter from the Teenage Mutant Ninja Turtles universe is an anthropomorphic rat who learned the ways of ninjutsu from his owner and master, Hamato Yoshi. He is the Turtles' sensei and adoptive father. In the 1987 TV series, 2012 TV series, and Archie Comics series, Splinter was Hamato Yoshi mutated into a rat instead of being just Yoshi's pet.
In Harry Potter and the Prisoner of Azkaban, the reader learns that the young wizard Ron Weasley's pet rat, Scabbers, is actually an animagus (one who can transform into a particular animal) named Peter Pettigrew. Pettigrew learned to become a rat when the friends of werewolf Remus Lupin became animagi to ease the pain of Lupin's lycanthropy, and later spent twelve years in rat form after faking his own death.
Rat is a 2000 Irish/British/American comedy film, focusing on the transformation of a working-class man into a rat, and how his family copes with the change.
The "were-rat" is also one of several animals that comes back to life by becoming monsters in Tim Burton's Frankenweenie (2012). It is explicitly named as such on the promotional cards given out in theaters.