Specially bred rats have been kept as pets at least since the late 19th century. Pet rats are typically variants

The fancy rat is a domesticated  brown rat (Rattus norvegicus), which is the most common type of pet rat.     The name fancy rat derives from the idea of animal fancy or the phrase, "to fancy" (to like, or appreciate).  

Fancy rats have their origins as the targets for blood sport in 18th and 19th century Europe. Specially bred as pets since then, fancy rats now come in a wide variety of colours and coat types and there exists several rat fancy groups worldwide. Fancy rats are commonly sold as pets in stores and by breeders. In fiction, pet brown rats tend to be depicted as tamed rather than domesticated, akin to when a character befriends a wolf. As tamed pets, they have played roles that vary from evil, to ambiguous, to lovable.    They can also learn simple tricks, such as learning their name, such as a dog, explaining their nickname, Palm Puppies.                

Domesticated rats are physiologically and psychologically different from their wild relatives, and—when acquired from reliable sources—they pose no more of a health risk than other common pets.    For example, domesticated brown rats are not considered a plague threat   , while exposure to wild rat populations could introduce diseases like Salmonella into the home.    While fancy rats are subject to different health risks than their wild counterparts, they are consequently less likely to succumb to other illnesses prevalent in the wild.

 of the species Brown rat, but Black rats and Giant pouched rats are also known to be kept. Pet rats behave differently than their wild counterparts depending on how many generations they have been kept as pets.     Pet rats do not pose any more of a health risk than pets such as cats and dogs.     Tamed rats are generally friendly and can be taught to perform selected behaviors.

The origin of the modern fancy rat begins with the rat-catchers  of the 18th and 19th centuries who trapped rats throughout Europe.     These rat-catchers would then either kill the rats, or—more likely—sell  the rats to be the victims in bloodsport.     Rat-baiting  was a popular sport until the beginning of the 20th century, it involved filling a pit with several rats and then placing bets on how long it would take a terrier to kill them all. It is believed that both rat-catchers and sportsmen began to keep certain, odd-coloured rats during the height of the sport—eventually breeding them, and then selling them as pets.        The two men thought to have formed the basis of rat fancy are Jack Black, rat-catcher to Queen Victoria, and Jimmy Shaw, manager of one of the largest sporting public houses in London. These two men are responsible for beginning many of the colour varieties present today.     

Rat fancy as a formal, organized hobby began when a woman named Mary Douglas asked for permission to bring her pet rats to an exhibition of the National Mouse Club at the Aylesbury Town Show in England on October 24, 1901. Her black and white hooded rat won "Best in Show," and ignited interest in the area. After Douglas's death in 1921, rat fancy soon began to fall back out of fashion. The original hobby formally lasted from 1912 to 1929 or 1931, as part of the National Mouse and Rat Club, at which point Rat was dropped from the name returning it to the original National Mouse Club. The hobby was revived in 1976 with the formation of the English National Fancy Rat Society (NFRS).       Pet rats are now commonly available in stores and from breeders, and there exist several rat fancy groups worldwide.

While domesticated rats are not so far removed from their wild counterparts as to justify a distinct subspecies (compare Canis lupus familiaris and Capra aegagrus hircus), there are several significant differences that set them apart, the most apparent of which is colouring. While random colour mutations  may occur in the wild, these are rare—most wild R. norvegicus are a dark brown colour, but fancy rats may be anything from white to blue.   

Behaviourally, pet rats are tamer than those in the wild.     They are more comfortable around humans, have decreased reactions to light and sound, are less wary of new foods, and can better tolerate overcrowding, though some rats perfer to be alone. They are shown to mate earlier, more readily, and for a longer period of time over their lifespan.     Also, domesticated rats exhibit different behaviours when fighting with each other. While wild rats almost always flee a lost battle, caged rats spend protracted amounts of time in a belly-up or boxing position.     These behavioural traits are thought to be products of environment as opposed to genetics. However, it is also accepted that there are certain underlying biological reasons for why some members of a wild species are more receptive to domestication than others, and that these differences are then passed down to offspring.       

At a physiological level, while still the same species, domesticated rats have different average statistics than wild rats. The chief difference is lifespan. Because domesticated rats are protected from predators and have ready access to food, water, shelter, and medical care, their average lifespan is around 2–3 years, in contrast to wild R. norvegicus, which average a lifespan of less than one year.     However, wild rats generally have larger brains, hearts, livers, kidneys, and adrenal glands than laboratory rats.     The fancy rat and wild rat also both face a multitude of differing health concerns—the former is at risk of developing a pneumococcal infection from exposure to humans, while the latter may harbour tapeworms after coming in contact with carriers like cockroaches and fleas.

As in other pet species, a variety of colours, coat types, and other features that do not appear in the wild have either been developed, or have appeared spontaneously. Any individual rat may be defined one or more ways by its colour, coat, marking, and non-standard body type. This allows for very specific classifications such as a ruby-eyed cinnamon rex berkshire dumbo.  

While some pet rats retain the "agouti" colouring of the wild brown rat (three tones on the same hair), others may be black based colours (a single colour on each hair). Agouti based colours include agouti, cinnamon, and fawn. Black based colours include black, beige, and chocolate.     Additionally, eye-colour is considered a subset of colouring, and coat-colour definitions often include standards for the eyes as many genes which control eye colour will also affect the coat colour. The American Fancy Rat and Mouse Association (AFRMA), a United States-based club, lists black, pink, ruby, and odd-eyed (two different types) as possible eye-colours depending on the variety of rat shown.     Ruby refers to eyes which normally appear black, but are shown to be red under bright light. Colour names can vary for more vague varieties, like lilac and fawn,     while the interpretations of standards can fluctuate between and even within different countries or clubs.