It's one of those "Well Known Facts" that isn't really much of a fact. Horseback riding, tampons, general athleticism, the splits, none of these are actually very likely, even when done for years, to have that effect.

Regrettably, deeper researcher will yield little proof either way, simply speaking, it can actually happen, but specific cases that are known are rare and typically suspect. Typically something that does will be very obvious and memorable, like landing on the saddle horn during a gallop, which is unlikely to happen to most riders even during prolonged and regular riding. To give you an idea here, anything that could tear a hymen should more easily result in tearing a number of other things on a man or women. I have a number of links from medical sources opn the matter, saying 'no', but by and large it is such a "Well Known Fact" that it's easy to find 'proof'. One source cites out of around a hundred cases, the only one they found was a toddler who tore it doing the splits, and otherwise it was basically the 'fell down the stairs' or 'got hit by the door/baseball' equivalent to cover up sexual abuse or juvenille experimentation. An appropriate analogy I think, as someone who has fallen down the stairs many times and gotten a black eye from a door and a baseball. It can happen, it usually did not.

Ultimately, as research shows, the major reason young women don't gush blood on night 1 is simply that most normally formed hymens will not tear with a certain amount of lubrication, natural or artificial, and that they do simply because the female participant is nervous and tense, big surprise there. One study I saw said 57% reported no bleeding, 43% did, odds are if that study was biased, it was probably from people reporting yes when it was no.

Ironically, because of it's cultural significance, many a young bride-to-be was advised to prick her finger on the wedding night 'just to be on the safe side' possibly with a little wink and nod, and of course riding a horse simply gave them a valid excuse when nothing happened.

So, not exactly a myth, not exactly, but kind of a "Chicken Soup is good for colds" sort of thing. Not false, just widely exaggerated.