Horses in dreams usually carry themes of drive and raw personal power -- the sense of momentum you feel, or wish you felt, moving through life. A horse running free can reflect ambition finding its stride, while one that's tethered, injured, or hard to control often points to energy you haven't found a healthy outlet for. Riding a horse well usually signals confidence in directing your own force.
The Moon is in conflict with those who are ruled by Mars, and Mars is in conflict with those ruled by the Moon. - Picatrix (Andalusia, ~1000.AD)