The central figure in "Ariel," other than the speaker, is Ariel the horse. Ariel is strong and fast, and, for some unknown reason, she sets off at a gallop and brings the speaker along for a wild ride through the countryside. By the end of the poem, the speaker becomes "at one" with the "drive," or the will, of the horse. In a way, the speaker becomes Ariel. So, for a terrifying near-death experience, this turns out to be pretty cool, if we do say so ourselves.
Line 4: The speaker describes Ariel in fierce terms, as "God's lioness." ROAR.
Lines 6-9: We see Ariel through her movements—through her pivoting "heels and knees," through "the brown arc / of the neck" that the speaker cannot catch. We get to know and understand Ariel through the small, quick glimpses of her body in motion.
Lines 26-31: At the end of the poem, the speaker becomes "at one" with the horse. She stops struggling against her and, in a weird and crazy way, becomes her. By acknowledging that she has no control over the horse, she is able to summon an even greater power, becoming "at one" with the horse's drive. Neato.