It's definitely a rowel and the oxidation doesn't appear to be iron oxide except on the pin. Any idea what kind of metal it's made of? There appears to be something stamped just to the upper-right of the pin... You might look at the whole thing closely under 10-30 power and see if there's anything to give you a hint. Otherwise, I'm unusually devoid of ideas. A lot of people collect spurs these days, so you might find something on a Google search.
There were a lot of toy spurs floating around in the 1940s and 50s when Roy Rogers and Gene Autry were at their pinnacle. If that happens to be pot metal you might have a toy spur, which would explain the fact that it really doesn't appear to have ever had the kind of points you'd normally see on a spur intended to be used for riding. I've found a number of old spurs through the years and I've never seen one as badly used as that one. My guess is that it's either a lot older than the 1920s or it's made of an inferior metal and is probably a toy spur.
There used to be a number of toy products of that sort for admirers of Roy Rogers and Gene Autry to allow them to show their loyalty to whichever hero they believed was 'King of the Cowboys'. I'd look for an RR, or Roy stamped somewhere on it.
Nice find.
J
A bit later: After doing a little magnifying on your scan it appears it's certainly either an ornament or a toy spur. Maybe the sort of ornament you'd find somewhere to hang from the rearview mirror. It's plated and the metal's inferior. I posted some blowups on IGH, one showing a spot you might learn more from if you clean it off. How big is the thing? Normal sized, for a spur? 1/2 size? It looks almost too flimsy to be a toy spur.
