Horses can exist without riders. Originally horses were without riders. They is only existing in natural habitats and in our minds. Our thoughts are like horses without riders. They are moving free and autonomously. Horses have physical bodies and colors but thoughts don't. Thoughts evolve from the bottom of consciousness. But they don't keep the footprints in the sand like horses. Consciousness is like an ocean. It is vast, deep, versatile and beautiful. In deep water it is calm and quite. But in shallow water near to the coast it makes waves, froth and sounds. It is similar in mind also. Thoughts are very emotional and hectic when the mind is connecting with outside world through senses. Always our thoughts are outward and extroverted. Because it is bound with outside objects. Then you needs to ride the horse properly.