There are no horses because if there were, people would insist on being able to ride them, and that would be impossible to implement satisfactorily with the game engine. Additionally, we are constantly moving through terrain where no horse could actually pass. None of the previous Baldur's Gate games had horses for the same reason. And in Dragon Age: Inquisition, you are the only person in the world who can ride a horse, and your group has to share a single horse due to engine limitations; earlier DA games avoided the horse issue for the same reason.