Fancy rat
The fancy (or pet) rat (Rattus norvegicus domesticus) is a domesticated subspecies of the brown rat (Rattus norvegicus). The name "fancy rat" has nothing to do with the "fanciness" of their appearance but derives from the meaning of "to fancy," e.g. "to like": "I quite fancy her." Thus one who keeps pet rats is said to be involved in the "rat fancy."
The origin of the fancy rat is probably the ratcatchers of the late 19th century who trapped rats and were paid by town governments per rat, and who also kept certain rats for exhibition/gambling fights. They began breeding rats (possibly to collect more money from the towns, or because it was easier than catching new ones all the time), and became attached to some as they discovered how intelligent and trainable rats can be. Rats became more and more domesticated and people enjoyed them since they are easily bred, social, intelligent and clean pets. The first rat show was held in 1901 in England. Beatrix Potter, author of the "Peter Rabbit" series of children's books, had a white pet rat of which she was very fond.
As in other pet species, a variety of colors, coat types, and other features that do not appear in the wild have arisen in pet rats through selective breeding. Some pet rats retain the "agouti" (two tones on the same hair) coloring of the wild brown rat, but other colors available now include blue, silver, black, white, pearl, fawn/champagne/beige, and red/cinammon. In addition to solid colors and the "Berkshire" pattern (colored top, white under), there are many different markings, including one similar to siamese cats (with seal or blue points), merle spotting, Dalmatian spotting, blazes, hoods (darker color on the head, shoulders and spine, generally a white body), caps (dark only on the head), and masks (only around the eyes), and a recent mutation, "downunders" which have markings on the belly that correspond to those on top. Rex coats are curlier (the whiskers are also curled), and satin coats are extra soft and shiny; several different genetic traits can produce hairless (or partly hairless) rats. "Dumbo" ears are lower on the sides of the head than normal ("top") ears, and genetically tailless rats are called "manx" just like tailless cats (manx rats come in the same stumpy, rumpy, and rumpy-riser varieties as manx cats).
Rats make excellent pets for urbanites with small apartments, as they bond with humans and offer much in the way of affection and entertaining activity, but require less space indoors (and don't need to be taken outside at all). Though they do urinate occasionally when allowed to wander outside the cage, the smell and damage to flooring is nothing like that from cats or dogs. They do have a tendency to gnaw. They are less costly to feed than cats or dogs (a basic rat chow can be supplemented with many leftovers from the human table, as they are omnivores). Their veterinary care, however, is just as expensive.
Although rats are generally nocturnal, many pet rat owners have found their fancy rats to be "crespuscular" instead: most active for a few hours around dawn and dusk, napping for a few hours at a stretch between these times.