In a world where horses (or any other species for that matter) were the dominant species instead of humans, could they ride humans in the same way that humans ride horses in this world? Could the human bear the weight of the horse? How would the horse sit? And is there example in real life of creatures (except humans) using other animals as modes of transport>
Short answer: No (at least not if horse-world-humans are the same as earth-humans)
Longer answer: Still no
Adult horses weigh some ~1900-2200lbs. A human can apparently deadlift some 455kg (some 1000lbs). As I didn't find a number for carrying stuff on ones back, I multiply the deadlift by 1.5, which results in some 1500lbs if we're generous -> This is still nowhere near the weight of a horse..
How would the horse sit?
(a) That is the first problem - before even worrying about whether it is possible to lift a horse. Most people down on all fours won't even reach up to the lower parts of a horse. A very high saddle would be required and the higher, the more unstable.
And is there example in real life of creatures (except humans) using other animals as modes of transport?
Yes, see the following video. (click link, not picture)