Ṭawran and tāratan are accusative adverbial expressions meaning “at one time” and “at another.” Their pairing creates the rhythm of repeated, changing movement.
In khalfa al-zamīl, khalfa is an adverb of place and al-zamīl is its genitive complement; zamīl denotes a companion rider. The pronoun in bihi is interpreted with reference to the tail already under discussion. Syntax thus specifies location and motion.