Dogs are cats' most common natural predator in the wild.
This is not to say, however, that cats and dogs cannot live amicably together in your home!
Cats also may be afraid of other cats, of humans (if not properly socialized), and loud noices such as thunder, automobiles, and lawnmowers.
Fearful cats generally have dilated pupiles, flattened ears, a flattened or crouched body, and a fast breathing rate. If threatened further, they may hiss or growl, arch their back and puff up their fur, and may even attack.