Which movement would best describe a horse moving laterally while simultaneously moving forward?

A half pass is a movement in which the horse moves diagonally forward and sideways, resulting in lateral engagement while continuing to progress in a forward direction. This movement is typically executed in the context of dressage and requires the horse to bend its body while crossing its legs over one another. The emphasis is on maintaining a forward gait while shifting laterally, which is characteristic of the half pass.

In contrast, the other movements listed do not encapsulate the simultaneous lateral and forward movement. For example, a rein back involves the horse moving backwards in a straight line, while a side pass entails horizontal movement without any forward motion, and a rollback typically refers to a sharp turn in the opposite direction after stopping or slowing down. Thus, the half pass is distinct in its integration of both lateral and forward motion, making it the best choice for this question.