Count Orlok in Nosferatu and the Other Mother (also known as the Beldam) in Coraline are frightening monsters because they have evil natures and intentions and strong magic powers, and they are enveloped in an air of mystery.
In Nosferatu, Count Orlok is motivated by a powerful desire for human blood, which he can barely control and which eventually destroys him. He kills many people, including everyone on the ship he takes to Wisborg, and more in the town. He is in disguise when Hutter first meets him and is able to change his shape. His origins are mysterious, and he is supposed to have been created by the fallen angel Belial, which would make him almost unimaginably old.
The Other Mother in Coraline has several of the same attributes as Count Orlok. She is driven by a need to possess the souls of children and does not care that she destroys them in the process. She is a witch with strong magic powers and uses these powers for malign purposes. Like Count Orlok, she is able to change her shape and disguise her true nature.