I mean, they can have black fur, after all. And they're nocturnal.
First, the non-sinister aspects:
Poor vision. They are nocturnal, so they don't have great vision in the day time. A monster isn't going to be very scary if it keeps fumbling around and dropping its glasses.
Poor appearance. Yeah, they don't really look very scary. Not very much at all.
Slow. Not as slow as turtles, but not really in a hurry to get anywhere; and its a good thing too, they can't run fast and can't really keep up speed for long.
Now for the sinister aspects:
Strong. Extremely strong. Very muscular.
Powerful claws. And hands like human's hands, with five digits.
Cubical crap. Yes! I'm serious. It's scary, how their poop is cube-shaped.
Amazing Burrowers. Wombats are the largest burrowing rodents that are still alive today.
Nocturnal. Hey, their night-dwellers and live underground most of the time. 'Nuff said.
A huge, monstrous wombat would not terrify the world. Its skin isn't thick enough to withstand missiles or anything like that, and everyone would think 'Ohhh, how cute!' However, a huge horde of red-eyed, black-furred, beastly wombats that come out every night to eat people would probably be enough to scare the cubical scat out of people.