Imagine an army invading on the back of hippos. But horses and sheep were easily domesticated. The reason for this is that they both need a leader and, this is important, will accept a human for the job. This is not always easy of course. Sometimes animals resist accepting a human as boss.
Some animals imprint on humans easily.
Others find this at first absurd. There was a show on television this past week about a real Horse Whisperer. There really are such people, apparently. My friend in Wyoming, Dexter Smith, has some of the mysterious ability to work with horses, which can be a very skittish and unpredictable animal. The original horse whisperer who inspired the recent book of that name is Monty Roberts.