Dead Red Fox

Grim circumstances aside, you've hit on a really interesting question, and one that tests just what it means to be a "native" or "non-native" species. In North America, some red fox are considered "native" and others "non-native." The big difference is habitat. Native red foxes occur in high-elevation habitats and in northern North America. Think Canada and the Rockies, though there are other places too. Those foxes crossed over to North America during various Ice Ages. The red foxes you'll typically see around people (including the one you found) are most likely non-native. They have their origin in European settlers who brought them over (probably to hunt).

If you want to know more, I found a couple links to articles, one from the Forest Service, and the other from the Wildlife Society Bulletin, that go into more detail.