Rats are non-fictional terrestrial rodent mammals native to Earth, where they exist in vast numbers, having permanently colonized every continent except Antarctica. Feral rats live in the shadow of virtually all human societies, opportunistically scavenging from humans' food, garbage and waste while also sometimes functioning as vectors for the spread of certain human diseases. Because of this, many human cultures have come to associate rats with infestation, uncleanliness and pestilence in general. Most major cities, like New York City, have large feral rat populations; humans usually consider their presence undesirable, so city rats spend most of their time in environments humans rarely visit, such as sewers, storm drains, inside the walls of buildings, and so forth.
Rats are also recognized for their intelligence and resourcefulness and even their capacity for affection. They can be bred and raised as pets (called fancy rats), and rats raised as pets in a clean well-cared-for environment tend to be healthier, grow larger and live longer than feral rats. The Mirage and 2003 TV series versions of Splinter began their lives as Hamato Yoshi's beloved pet fancy rat. Rats are also important to scientific research in the form of laboratory rats where they function as similar analogues to human biology. This is because humans and rats (along with colugos, lagomorphs, treeshrews, other primates and other rodents) are more closely genetically related to each other than they are to any other mammals, and all these related species are taxonomically classified together as the clade Euarchontoglires, also known as Supraprimates. The IDW version of Splinter is the reincarnation of the human Hamato Yoshi, but was born into his new rodent life as an otherwise ordinary laboratory rat at StockGen.
Certain other recurring fictional characters and technological inventions in Teenage Mutant Ninja Turtles are associated with rats.
Most versions of Baxter Stockman invented robotic machines called Mousers, whose primary function is to hunt and eat rats (making them a danger to rat characters like Splinter), but can also be directed for more insidious tasks like assassinations, bank robberies, the intentional collapse of city buildings, and mayhem in general.
The Rat King is also strongly associated with rats. Most versions of this character have a supernatural influence over rats, having the ability to compel them to swarm specific areas or against specific targets. Some versions of the Rat King can use this same power of persuasion to compel ordinary rats to be his eyes and ears, acting as spies and informants. However, most versions of the Rat King are themselves actually humans rather than rats.