The rhythm of the sentence wonderfully echoes the rhythm of the ride -- to me, this line is a highlight in one of the best-written passages of the novel.

There's also a possible symbolism to Don Pepe's horse: with its brutal disposition and its "hammer head," restrained and steered by silver (desire), and as fragile at the neck as a female sheep, it could represent the brutal yet fragile State, and the rhyhmic "rising and falling" of the ruler on its back could represent the political cycle of State-forming and State-falling, which Nostromo illustrates. Note that Don Pepe, who represents the State, is "hardly visible in the rear"; we are in Charles Gould's ideal of material progress here, which overrides the State it makes use of.