* The fictional Superman: The Superman of the comics, movies, and TV shows never fell off a horse. He's known for being super strong and incredibly agile.
* The inspiration for Superman: Superman's creator, Jerry Siegel, said he was inspired by the "Amazing Stories" magazine, specifically a story called "The Man of Steel," about a man who gains superpowers through a meteorite.
So, in the fictional sense, Superman didn't fall off a horse. But, the story that inspired Superman was published in a magazine known for its stories about "men of steel" who could do extraordinary things, which could be seen as a metaphorical "fall" from a "horse" of normal human limitations.
Let me know if you want to explore any of those concepts further!