Why does cats raise their fur when they are scared?

There are several different reasons why a cat fluffs up his tail.

When a cat is in a confrontation situation, he will fluff up his tail - and the fur on his entire body also. What he is in fact doing is trying to make himself look "bigger" in front of his opponent.

If a cat's tail is lowered but fluffed up and he is adopting a crouched kind of position, this generally indicates that he is frightened of his opponent and does not want to enter into any kind of confrontation.

Sometimes when a cat is in a playful mood he will fluff up his tail, but in this instance you will notice the tail is in an upright position and he generally will not give the appearance of being frightened.