I found this on the "About Eventing" page of the Rolex Website: The ultimate test of horse and rider! One event, three disciplines, testing the resolve of the timeless partnership between horse and rider, to trot, gallop and soar past anything that stands in their way.
Not totally poetic but might be helpful to you as a decent summary quote.
Here are three that I think describe the feeling of galloping cross country:
When God wanted to create the horse, he said to the South Wind, "I want to make a creature of you. Condense." And the Wind condensed. Emir Abd-el-Kader
In riding a horse, we borrow freedom. Helen Thompson
Horse, thou art truly a creature without equal, for thou fliest without wings and conquerest without sword. The Koran
"We can judge the heart of a man by his treatment of animals" Immanuel Kant