I'm pretty sure my flist has it: it's some kind of seal fobwatch ornament - none of these examples look exactly like it (or each other), but you can see a couple have exactly that kind of arch, and if you also Google image seal rings, that looks like the sort of moulding or engraving on the 'coin', too. And it fits with the watch chain & why it seems kind of normal. (Probably other people have had some kind of fobwatch ornament included in a costume - it just wouldn't usually be pulled out and used.)

ETA: And if you put in "spinner watch fob", these are all a pretty good match (if, again, all very individual) here. So it's definitely an ornamental watch fob, probably using a coin or seal, and I think the most correct term is just "watch fob" or "ornamental watch fob" or "spinner watch fob". (I said my flist knows everything! Heh. :-D)