Native to the Gray Waste, skinrats superficially resemble Cranium Rats (that is, small, ratlike, and with brain matter poking out of the tops of their skulls). Unlike cranium rats, skinrats are hairless and not at all psionic (and not connected to Ilsensine). The peculiarity of the skinrat is that their sweat acts as a reduce spell cast at 5th level. You can get the full effect just by licking it.
Source: In the Cage: A Guide to Sigil, pages 102-103