Believe it or not, the issue has just come up. The Wisconsin Court of Appeals concludes that it indeed probably isn't a crime (or at least not the crime charged), but that the defendant waived the argument by pleading no contest:
Hathaway first argues his conviction should be reversed because the term “animal” in WIS. STAT. § 944.17(2)(c) does not include an animal carcass. He rather convincingly contends that “animal” means a living creature.
However, Hathaway pled no contest to the charge. A plea of guilty or no contest waives all nonjurisdictional defects and defenses.... [Hathaway's] argument that having sex with a dead deer does not violate the statute is a nonjurisdictional argument. It does not go to subject matter jurisdiction. Consequently, the argument was waived.
Thanks to How Appealing for the pointer.