As a young 13 year old, I learned early on that horses have amazing memories. With my mothers help, together, her and I worked long hours to gain the trust of a wild mustang.  His name was "Blue" and he would go ballistic when a man would approach him.  A few years later, we found out that some men tried to "break" him out and Blue actually tried to kill them.  It was amazing to me how he held this grudge his whole life.  A man could never brush, lead or ride him; however, he loved women and children.  I know the research shows that when a horse has had a positive experience, they will remember things for a very long time, if not, a lifetime.  My experience has been both ways. Good or bad, horses do remember.  Check out this article, http://news.discovery.com/animals/horse-friends-memory-trainers.html