In Finland, we call everything "it" in colloquial language. We don't discriminate. (or at least we discriminate everything equally)
But to your question: Corpses are "it", but deceased persons aren't, because you're still referring to them as a person instead of a corpse.
I would personally call pets "it", but I guess it's a sentimental thing for most people. Rooster already pointed out the deal with babies. Have you ever heard anyone call a baby "it" after its sex has been determined?