The galloping horse
Horses represent vital energy, and what Freud called the libido. A galloping horse shows a strong desire to make the most of life, which can be a positive and motivating force. It can also be interpreted as a need to break free from restraints, or a sign that you are at a sexual peak.
If the horse bucks however, or has an accident, it may mean that the dreamer has taken on more than they can cope with, or is unrealistic about their abilities. Be aware of your limits, pride comes before a fall...
Horse riding
Horse riding in a dream is a sign of a strong life force. This is very positive and means that the dreamer is making progress in a certain area, that they have taken control of their life. Maybe your sexual energy is particularly focussed, or you are motivated to reach your objectives in many different areas.
Wild horses
Wild horses are a sign of very powerful instincts or feelings which could sweep away everything in their path. This may be sexual, you may be about to experience an extremely strong attraction which defies reason.
They also represent life's natural rhythm and power, meaning that the dreamer could soon find themselves able to overcome obstacles that they had thought were blocking their way.
Being trampled by horses
If you are kicked, or trampled by horses in a dream, it symbolises unchecked sexuality, or even sexual obsession. This may be putting the someone's psychological wellbeing at risk, either the dreamer, or someone they know. It is often a sign of incest, or an inappropriate relationship which may lead someone into danger. You have been warned!