Rats are a common species of rodent. Members of this race have appeared in every iteration of the Sam & Max franchise to date. They have appeared both as intelligent and non-intelligent creatures. Some rats appear around the same size as their real-world counterparts, whereas others are the proportionate size of humans. Rats have appeared both on the Earth and on the Moon.
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In most comics, the rats are background characters in scenes, seen stealing food out of trash cans or running across a panel. However, the moon rats in Bad Day on the Moon are the size of Sam and are important to the plot.
Steve Purcell had pet rats, as mentioned in Telltale's article History of Sam & Max part 1. One was named Ernie, who inspired the rats in Steve's other comics. Toybox, another comic by Steve, has a rat named Ernie, likely named after his own pet rat.
A majority of rats in the series have black or darker fur, with some having white patches on their chest. There is a group of rats in the Telltale series of games known as the "skinbodies", who shave the fur off of their torsos (though they leave their head, and occasionally legs, untouched).
Several unnamed rats appear in the walls of Sam & Max's office from Ice Station Santa onward. Most are nondescript, though there are two - a large dark-furred one and a smaller white-furred one - meant to represent Sam and Max, with the larger rat having a hat that resembles his.
In real life, there are many species of rat, though one of the most popular is the domesticated Fancy Rat. Fancy rats come in a variety of colors, usually some variation of white, brown, black or red, and patterns. It is probable, given their markings, intelligence and potential inspiration, that the majority of rats seen in the Sam & Max series are based on domesticated rats.