Well, let’s start with a few questions and assumptions.

Question: does a Necromancer’s control of the dead work like mind control? I.e. it moves under its’ own power, but at your direction? Or does it work more like a golem, where it’s movement is entirely controlled and held together by magic?

Assumption: Let’s assume a taxidermied animal has had its organs, bones, and muscles removed. Even if it were controlled, could it still movie? It depends on if Necromancy is closer to a golem spell, or a mind control spell.

Assumption: But there’s another issue to consider. Common folklore is that, removing a zombie’s head will kill it. So let’s assume that’s true.

Question: If a zombie “dies” when it loses it’s head, is this due to it losing it’s brain?

Conclusion: So, I’d say that common necromancy theory is the body’s spirit is returned to the body, and that the spirit is controlled by the necromancer. Further, a zombie “dies” when it loses it’s brain. Therefore, since a taxidermied animal has no brain, it cannot be zombified.