in my country (belgium) there arent a lot of mythological creatures but the one that pops into my mind when i read this thread is the legend of kludde. Supposedly he's some kind of demon that takes on the form a big black dog that walks on his hind legs and carries a chain around wich he uses to scare people. He counts as a water demon that dwells under bridges, hollow trees and water pools where he hides and waits for people to pass by and then jump onto their neck and doesnt let go until the person has reached their destination. Other than that he sneaks around farms in the middle of the night rattling his chain just to get on your nerves or freak you out.
http://www.pantheon.org/areas/gallery/folklore/folklore/kludde.html