If you go to the page of the horse itself you'll see 3 buttons on the right side near the arrow button.  One of those is a circle with a line through it.  You click that, it allows you to edit the horse.  On that editing page you'll see something called Working Paths with four check boxes.  Check off the one that says discipline.  Once that is checked look underneath for the box labeled Primary Discipline and select the one you want from there.  Then save the changes by going back to the top buttons and clicking the box with the check mark over it.