Which preposition to use with bareback

on Occurrences 2%

Even then, the term sumptuousness may seem ill-chosen, since the nomadic nature of African life persists in spite of palaces and chamberlains and all the elaborate ritual of the Makhzen, and the most pompous rites are likely to end in a dusty gallop of wild tribesmen, and the most princely processions to tail off in a string of half-naked urchins riding bareback on donkeys.

at Occurrences 1%

Judith rode him bareback at a gallop down to the swimming pool and dived from his back into the yellow water shimmering hotly in the sun.

of Occurrences 1%

They had seen Red Jim come battered and exhausted from his struggle with the stallion the day before, and now he sat upon the bareback of the chestnuta miracle!

Which preposition to use with  bareback