Hi,
There may be expressions for this, but, can’t help with those.
If your only using After Effects and no true 3D programs I think you are going to be stuck with distorting it. How you go about that depends on exactly how you want it to flap at the tail. Flattening and waving is a bit different than actually seeing part of the back.
If you have Zaxworks then go with that. If not then create a looping displacement precomp with something like Generate->Radio Waves and have it only affect the tail. The speed stroke and blur of this radio wave precomp will all greatly affect the outcome, so play with the sliders until you are happy.
Alternately use Distort->Wave Warp on a masked portion of the tail.
You can even go for the puppet tool, -it’s always fun- set a couple of pins in it and start puppeting =)
Hope that gets you waving -or flapping-,
Ben