Hey Matthew,
since you’ve created the animation in flash, all you’ve got to do is export it as a swf and import the file from after effects and just loop it.
However, if you still want to do animation manually in after effects the best way would be is to go with shapes (if your not looking for a 3d kind of depiction) and have the tail animate over a period of time (so it can be easily looped with some sort of variation)
To add variation you can simply plugin some kind of distort / warp filter and plug in a random expression to self animate it over a period of time.
Hope that helps!
Cheers!