A Horse is listed as having the following attack Melee 2 hooves –2 (1d4+1), this would indicate that it is a secondary attack.  But the rules state “If a creature has only one natural attack, it is always made using the creature’s full base attack bonus and adds 1-1/2 the creature’s Strength bonus on attack rolls.”  So this means the Horse has a primary attack not listed probable bite or the listing is a error.