The Horrible Thing That Screams in the Night.
Really, I don't know what it is. It sounds like a mixture of a yowling cat, a screeching bird and a whinnying horse, with sometimes a touch of demonic laughter thrown in, and sometimes a sort of whimper. I think it can fly. It's the most horrible thng I've ever heard and if I didn't know better I'd say it was a
literal denizen of Mordor.