Do we have a tag for this kind of wing?

Wings that do not have matching counterparts.
For example, Houjuu_Nue has two different "wings", and Sephiroth sometimes has a single wing.

Or this: post #506397

asymmetrical body, perhaps?

If you wanted to be specific, could call it asymmetrical_wings.

Would having one wing being a sub-category of this?

I think one_wing should be a tag in the case of Sephiroth.

hidetheunforgiven said:
I think one_wing should be a tag in the case of Sephiroth.

It is.