Taz42o is correct, as usual. Specifically, it is an antique automobile tire valve stem cover. They come in various shapes, and have been discussed here in the What-Is-It forum several times. Here's a photo of one version of antique tire valve stem cover, on a Model-T Ford tire, posted previously by BigCypressHunter.
If you are willing to do some additional research, you might be able to discover the exact version (Chevrolet, Ford Model-T, Buick, etc) and time-period (1910s, 1920s, 1930s) of the one you found. I notice that yours has multiple flat sides, which will make it easier to exactly identify.