A man on a horse is not a man or a horse to a lion.
It is a single, strange animal.
The shape is tall and wrong.
The scent is a mix of horse and human, a smell the lion does not associate with a meal.
For the most part, this unfamiliarity is the tourist’s safety. The lion sees this new shape, but does not recognize it as prey. It is not a zebra or a buffalo, this is why horseback safaris can exist.
But safety is a fragile thing. If the tourist falls, the picture changes — The single strange animal. Becomes two known things – A man, and a horse. A lion can kill a horse, and a man on the ground is weak — The guide’s skill is what a man pays for. Keep the illusion intact.
The lion leaves the horse alone because of the man. Without the man, the horse is just meat.