The statute forbids sex with an animal. Since the deer he had sex with was dead, and since "animal" implies a living creature, he cannot be convicted (so argued his appellate lawyer).
It looks from the opinion that the appellate court was somewhat convinced by that argument. However, it still upheld the conviction on the grounds that the man pleaded "no contest" to the charge, which means that he basically admitted guilt to the words of the statute, whatever they might mean (dead, alive, etc.)
Deer in Wisconsin celebrated the court ruling. News story here.