First of all, is the thing real? If it is, animals are usually numbered or tagged as part of a research study, or because they are endangered. You didn’t indicate what part of the country/world you’re in, you might want to google your area and “alligator study” to see what it might be from. In some parts of the world, there are species of alligators that are endangered…and to have a decapitated one in your garage might not be the best thing.

If you don’t live where they’re endangered, you’re sure the thing is real, and you can’t find any studies in your area, I’d suggest calling animal control to see what they make of it.