Grand Theft Horse

I was looking through some of the collected papers of William Henry Harrison, and found this gem (scroll down to bottom of pg. 80). It speaks for itself: a suspected British agent infiltrated the American post at Vincennes, Indiana Territory and made off with Colonel Decker's horse, together with its saddle and a fine pair of pistols. I'm hoping to find out how this all played out...
From "Messages and Letters of William Henry Harrison Volume 2"