This poem is famous for its onomatopoeia, a device that helps to bring the speaker's exhilarating ride to life.
The most obvious (and insistent) onomatopoeia here is the speaker's use of the words "galloping," "galloped," and even "galloper." These words themselves have a galloping rhythm and evoke the clop of hooves with their /l/, /p/, and /g/ sounds. Hardly a stanza goes by without a variation on "gallop"; some version of the word appears no fewer than 13 times! Galloping onomatopoeia thus underpins the whole poem, making the verse sound just like the wild horseback ride it describes.
Elsewhere, certain words work like sound effects—as when the speaker describes the horse Roos's exhausted "wheeze" as she collapses, or the "spray" of mist around the horse Roland as he (you guessed it) "gallop[s] on."
These little moments help readers to hear the speaker's ride as well as envision it. When the speaker remembers the "bright brittle stubble" that "broke" under his horse's hooves, for instance, readers can both sink into the visual imagery and hear the stubble crunching in all those /b/, /r/, and /t/ sounds.
Onomatopoeia is, in short, a big part of what makes this poem fun. These evocative sounds help readers to feel like they're right there with the speaker, riding along on this adventure.