The idea of a "monster" revolves around the physical embodiment of fear. If no description can be provided for the thing that causes fear, then what's to fear?
If you really want the reader to imagine and fear the monster within the story, you have to go ahead and give the monster characteristics that would scare and make people fear it. You have to use specific vocabulary to describe it.
Example: As soon as the monstrous beast bared is blood-stained fangs at the Knight..etc
So you see what I'm trying to say, Use big words and use your imagination!
