In addition to the above, for me what makes the mechanical hound seem so menacing and destructive is that, for the most part, it can't be stopped.
A real dog, no matter how large and ferocious, is not as indestructible as the mechanical hound seems to be. A dog can be fought against, at least. A rock or a big stick, whatever, can ward off a dog; not to mention a piece of meat. But the mechanical hound does not stop, will not back off, will not lose interest if you show submission. The mechanical hound leaves a human with no options. That's scary.
Except one, of course: fire. But unless you happen to have a flamethrower with you, you're out of luck against a mechanical hound--or at least that's how it seems.
Finally, the mechanical hound itself is not evil, since it's a machine. The people who program it are evil, but the hound itself is not.
I generally agree with the first answer, although I would point out that the first time Montag sees the Hound it is "asleep." So don't write that it is walking around and it snarls at him.
I think I would add a couple of things to the first answer. First of all, the Hound has 8 legs and that makes it seem like a spider. Spiders are scarier to most people than dogs -- their body shape and the way they move is.
Also, look at the exact words Bradbury uses to describe this thing. His first words about it are
The Mechanical Hound slept but did not sleep, lived but did not live...
So right there, it already sounds evil -- like a vampire or something. Then he goes on to say
It was like a great bee come home from some field where the honey is full of poison wildness, of insanity and nightmare, its body crammed with that over-rich nectar and now it was sleeping the evil out of itself.
To me, this makes it seem evil.
Finally, in terms of destructiveness, I would look at that game they play with it where it runs around killing all the animals. That just shows it destroying everything it is asked to.
The physical description that Bradbury gives of the mechanical hound makes it seem evil, menacing, and destructive. In the beginning of the novel, Montag sees the hound coming around the corner and it snarls at him like a real mad dog. The glowing eyes and creeping posture attributed to the hound also serve to develop its evil appearance. The hound is sent out to destroy books and keep citizens in line, so it is always on the hunt for people who aim to usurp the system. So it is a menacing, destructive element in the story.