I'll type fast in the hope of getting a word in before tsuwm, so forgive typos.

I think a necropsy is the postmortem examination of any creature, and that, if autopsy is really understood to mean a postmortem exam of a human, it would be a particular kind of necropsy. Necropsy may have just been adopted by the vet trade.

Oddly, in my little desk dictionary, autopsy is defined as "the act of seeing with one's own eyes." Perhaps it's related to seeing with one's own eyes how the deceased really died, rather than trusting the widow who collected on the huge life insurance policy or the guy with the smoking gun running away screaming "Finally, finally! Mwahahaha..."