A cockroach as big as your fist?
That mouse you chased, back with an adorable chainsaw?
A thumb-less hand?
Your water pipes reforming into a steam-powered engine of penetration?
A callous squirrel?
The end of days?
A fat rabbi reading excerpts from Bridge to Terabithia?
One joke too many, skewing the entire legitimacy of your silly list?
Short Answer: It's probably just whatever used to live in your closet or under your bed. We've figured out a lot of its methods, but talons allow you to move easily through the ducts.