1. Draw the tail mask on a solid, using the pen tool in the comp window.
2. Hit M to reveal the mask shape property in the timeline.
3. Set a key for mask shape at that point in time.
4. Move to another point in time.
5. Using the arrow tool, select mask points and drag them to new positions. A keyframe for mask shape will be created automatically.
6. Preview and repeat until satisfied.
To see the tail, apply render>stroke to the layer.
There should be more in the help.
Steve