I think what makes a monster scary is it not being seen. Just knowing something is there lets the imagination run wild. Never truly knowing what is coming for you is immensely scary. Another key point is that it has to be incredibly strong. This means that it should and will be able to break through or destroy anything and everything standing between it and you. Inescapability I think is the word. Finally, when the creature does eventually show itself, it should be grotesque. Typically this means taking the basic human figure; male, hairless, bipedal; and making slight alterations. Arms that are long enough to touch the floor staring straight up. Elongated neck and jaw and lifeless eyes. Etc.