Perhaps not the best answer, but...

If we're allowing mystical or imaginary creatures, I'd definitely be a dragon. I've simply always been fascinated with them, and they're such magnificent and knightly creatures that one wouldn't be able to help but envy them.

Assuming they're off limits, I'd be a...oh, I don't know, winged animals generally aren't very interesting. Maybe a sparrow or some small songbird, they seem to be so carefree when you see them singing on a spring morning.