One of the rarest (and most obscure) of mystical creatures is the fabled raticorn. This tiny creature is elusive and skittish, and only appears when tempted by the purest bits of cheese. She is the wisest of all rodents (which isn't saying much, really), and to catch a glimpse of her is surely a sign of good luck (if you don't mistake her for one of her more common sewer-dwelling kin and end up calling pest control).
The raticorn is sometimes hunted for her horn, which is rumored to have magical properties; when ground into a fine powder and consumed, it gives someone the ability to discern edible, juicy garbage from the stale, moldy kind. This ability is valuable particularly among dumpster divers, who are the raticorn's most notorious poachers.
Some skeptics doubt the existence of the raticorn, but the most patient and pure-hearted of us may catch sight of her disappearing into a sewer drain, or scurrying under the floorboards. One must only watch, and wait, and have some tasty cheese at the ready.