I have never seen one of these but it was purchased from a sale of military memorilbila.  The guy operates a museum and this was in his sale.
From the research I've done these are rare and hard to find,used to hold the reins on a horse, donkey or pony drawn buggy.
It is made of steel or cast iron with bronze rings and hook.
It measures 7 1/2" wide by 4" back to the edge of the saddle and 5 1/2" tall. I can't find any markings on it.
What a great find for the equestion collector.  Would look amazing in any decor.
Please see and magnify all photos for a better discription and a better look at its condition, feel free to ask any questions you may have.