As spotted in the barn tack room. It's hollow and rather lightweight. What type of bit is it, and what does it do?
(I'm quite sure it's being used with trainer knowledge and approval, at least if it does in fact belong to the horse whose bridle rack it sits on, which leads me to think it's a thoughtful attempt to address some issues with a tricky mare. This isn't an anti-gadget snark by any means.)