the back of a horse
"She adjusted her hat before mounting on horseback to ride through the fields."
The back of a horse.
"The rider slipped off his mount and scraped his knee on the hard ground beneath him after losing his balance on the horseback."
on the back of a horse
"he rode horseback to town"
"managed to escape ahorse"
"policeman patrolled the streets ahorseback"
On the back of a horse.
"The rider adjusted her grip on the reins before mounting the horseback to begin the trail ride."