I once had a deer mounted, that did not turn out as I expected.
Because taxidermy is an art, and art is subject to interpretation, I took the mount to an experienced friend. When I took it inside his place, the first thing he said was "I don't think I would have paid for that!"
I called talked to a very reputable, but very expensive taxidermist, he suggested two options:
1. Have the trophy examined by 2-3 licensed taxidermists, and get them to write a brief statement on why the trophy is substandard and get ready to go to "small-claims" court.
2. Explain to the taxidermist who did the sub-par work that you have had their work looked at by other taxidermist who agreed that the work was not up to standards. Offer to pay for the materials used by the taxidermist who did the sub-par work, but nothing more.
I took option 2 and he reluctantly gave me back half of what I paid.
I have since taken better deer, so the mount now resides in my "man-cave".
When I have buddies over, I sometimes clap my hands when I'm near it hoping it will start singing!