The answer is:

A Boxfish. it was his tail.

It seems this was quite a difficult one as hardly anyone of you lazy people aswered. And the few who did answer were waaaay off. Except for one person and this weeks winner:

I am of course talking about my fabulous cousin Lise M. Perceptive as she is she noticed that it had to be a fishtail and not, like some other people insisted, a shuttle cock. you know who I'm talking about...