Horse dreams symbolize vitality, freedom, and rapid progress. Riding a white horse and galloping is one of the best lucky dreams, meaning gaining honor and achieving swift success. A black horse indicates strong adversity but the power to overcome it. A rearing horse predicts a new opportunity arriving.
Animals appearing in dreams — as in "Horse Dream" — often symbolize instinct or a suppressed impulse in Jung's theory of Archetypes. The specific animal and the emotion it evokes in you both shape the meaning far more than a generic dream dictionary entry can.
What was the first emotion you felt when you saw that animal in the dream?