Skinwings
(Aeromys pterodermatus subterraneis and sub-families) Underground-dwelling flying rodents of the New York urban ecology.
There is a relatively widespread misconception that skinwings are some kind of bat. Actually, the Bestiarium Ignotum places the species in the sub-order Sciuromorpha, and the family Petauristinae, among the flying squirrels. The nearest "outer world" relative is Aeromys tephromelas, the black flying squirrel of southeast Asia.
No one is certain at what point in their evolution the skinwings went underground and changed from herbivores to omnivores. Remnants of what seem to be long-extinct colonies of an earlier furred variety in caves in Greece and Sicily suggest that the ancestors of skinwings might have moved underground in an attempt to survive the global winters caused by volcanic disasters such as the great Minoan eruption that resulted in the destruction of ancient Thera. Somewhere along the line, the fur coat was lost -- probably due to the sheltered nature and relative warmth of their preferred caves in the Mediterranean basin -- and the omnivory is probably due to the relative paucity of vegetable matter underground. Skinwings in modern urban "cave" settings do still willingly eat fungi like firefungus, but when given the chance, they seem to prefer live prey, probably because it provides significantly more energy per foraging cycle than vegetable material would.
As regards the issue of how skinwings got as far as North America, there are theories, but no positive answers. Hitchhiking in cargo seems the most likely possibility. In particular, caves in southern Italy and Greece have a long history of being used to store wine and food: the nucleus of a New World colony could easily have been established by the transshipment and storage (or loss) of just one crate or empty amphora in which skinwings were roosting (or, as during the cold season, estivating). The skinwings would have done the rest, escaping as soon as they felt it safe to do so, and seeking out the warmth and darkness of the New York subway system via one of any number of damaged grates or accesses.
Because of their subterranean environment, their very shy and retiring habits, and also their evil reputation (probably due to sicknesses caused by the strain of Coccidia they harbor), not much is known about the life cycles or habits of skinwings. Most wizards avoid them and their haunts for the same reasons people now avoid deer mice and ground squirrels: their droppings, when aerosolized, spread the hosted coccidium (Eimeria aeromysis) by inhalation, causing parasitic coccidial diseases like toxoplasmosis. (SYWTBAW)
(See also: Fireworm: Hidebehind: Mouse, Dun: Subterranean ecologies: Thrastle.)