To dream of an old horse or a 'nag' indicates a winding down of an idea or project and suggests humility and acceptance at this time.
A nag trying to keep up with other horses in a dream implies an inability to let go of a project but cautions you to release any expectations for your own happiness.
To dream of a person nagging you suggests that you may be trying to ignore that little voice inside that is trying to steer you in the appropriate direction. There is a reminder to stop and listen as you have the answer and you deserve to be heard.
To dream of nagging another person may indicate a personal frustration with an unconscious hint to relax and step away from a current bothersome situation.