Mouse, Dun
(Mus umbratus) A small, magically "mutated" rodent with a gift for not being seen.
Dun mice are thought to have evolved from the common house mouse (Mus musculus Linn.). House-mice were quick to move into the relatively sheltered subterranean habitats of the tunnels and crawlways beneath the streets of human cities. Subway tunnels were equally exploitable, especially since careless humans would often enough toss food and food containers onto the tracks. The many predators sharing this environment with them soon forced these mice to find any adaptation that would help them survive. Mice that were dull in color, or closely matched the dim shades of underground stations and trackways, lived to breed more mice with the same characteristics. Soon almost all mice living in such environments had come to closely match the dusty color of between-track gravel and cinders. These are the standard "subway mice" now to be seen in almost every underground transport system in the world: the London Underground estimates its own "Tube mouse" population at nearly half a million.
The dun mouse, however, did not stop its adaptation there. Since the first dun mice were discovered in the neighborhood of large worldgate complexes like the ones at Grand Central Terminal and London-Tower Hill, it is generally thought that high-level wizardly leakage from the worldgating facilities caused the genetic shift that allows dun mice to become invisible at will. Such mice quickly discovered that they had acquired an advantage over many of their predators which the ability to merely "blend in" couldn't match. There are some parts of some subway systems where dun mice have completely outcompeted and therefore displaced all other Mus species, and even some of the rat species (since any predators which do not hunt by smell or hearing then turn their attention to prey they can see rather than wasting their time on what they can't). In other places, where predators like cats are more common, the dun-mouse populations spend almost all their time invisible, and often can only been detected when they have been eating firefungus, and the telltale residual retinal glow gives their presence away in the dark.
Dun mice can still be seen with surprising frequency in their normal subway habitats, where they forage on the tracks between trains. Even nonwizardly viewers may spot them -- at least those who're willing to believe that they've just seen a mouse fade away to nothing or appear "out of thin air." (SYWTBAW)
See also: Subterranean ecologies: Unnatural history: Wizardly fauna (North America, Europe).