My guys (especially my 1.5yo male) wag their tails just before they attack -
the dog, the cat, each other.  It's especially funny to watch when the tail
is all you can see, ie, the rest of their body is under the couch, a
blanket, etc.  It makes you take notice and watch because you know something
is about to happen!  They also tail-wag when they are playing in their tube.
[Posted in FML issue 2376]