Peleg first tells us that men and horses are very much alike.  He then describes his horse Billy Lee who would stop before the finish line and rear back and throw his rider.  I think that what Peleg means by this is that sometimes when we near success we sometimes panic because of self doubt.  The horse stops and throws its rider in the same way many people act in self defeating ways.  Sometimes men are not best fitted for success and greatness even if they have what they need to achieve it.  Some men are like Billy Lee, and will refuse to work and refuse to race, refuse to take part in the insanity of a race or the mindlessness of labor.