White is the recurring colour in Scripture of heavenly beings (see also Matthew 17:2; Matthew 28:3; Acts 1:10). Furthermore, a victorious general would ride a white horse in his victory parade. Those two notions together suggest that this white horse is a heavenly being that is victorious.
2 And I looked, and behold, a white horse! And its rider had a bow, and a crown was given to him, and he came out conquering, and to conquer.