To make a monster really scary, there has to be something "wrong" with it. Something unsettling. Be it a strange walk, the tendency to slowly turn to face you before practically teleporting into your face, or a grotesque shape.
Other factors could be intelligence. IE, you can TELL the monster is messing with you. For example, you catch a glimpse of it around a corner, the when you come around, it's not there. then you turn back and the hallway you just came down has claw marks everywhere, and now you don't know where the thing is...But you know it's smart enough to be laughing at your nervousness right now.
Or, even more simply, you don't know where it is, but you KNOW that if it catches you, it will kill you in one single hit. Or summon millions of allies to smother you.
These are all things that make monsters scary.
Other factors could be intelligence. IE, you can TELL the monster is messing with you. For example, you catch a glimpse of it around a corner, the when you come around, it's not there. then you turn back and the hallway you just came down has claw marks everywhere, and now you don't know where the thing is...But you know it's smart enough to be laughing at your nervousness right now.
Or, even more simply, you don't know where it is, but you KNOW that if it catches you, it will kill you in one single hit. Or summon millions of allies to smother you.
These are all things that make monsters scary.