Black Dog:

Coat, blacker than night, this doesn't gleam in the moonlight like a coat of a living dog.

Eyes, some report eyes glowing of evil; others tell of terrible, empty black pits. And yet some others say blood-red.

Claws, these spring viciously from huge paws that make no sound as the monster lopes along. The paws also leave no prints, even in the softest soil.

Mouth, the teeth are hideously long and sharp, and foul saliva reeking of sulfur may drool from the beast's jaws.
And lets not forget its strength and agility, both of which are great.