A cat shows its emotions through its tail. When happy and confident, the tail stands straight up, but when frightened and scared, the tail often puffs out.
Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, said that cats are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment.”
By making themselves look bigger, a cat’s puffed up tail can (hopefully) frighten away anything threatening. So if you see a cat puffing up its tail, stand back. You may not feel like a threat to a cat, but you can be sure that a cat will always be a threat to you.