The Stallion that Mounts the World is an Apocalyptic Image of Self-Destruction

The Stallion That Mounts the World prophecy is an example of the theme of presenting weakness as strength. Probably the best example of that theme is the claim Strong Belwas makes that every scar on his belly represents a man he's killed since, as a pit fighter, he lets every opponent cut him once before he kills them. Scrutiny challenges that claim quickly, yet what better claim could a pit fighter to use to embolden himself? The Dothraki practice of wearing bells is another example of presenting weakness as strength. They are a people so poor that they must sack wealthier peoples to survive. The Dotrhraki can't even remain in Vaes Dothrak or they wouldn't survive. Khalasars must keep moving and collecting protection money or they starve. Swearing off stealth, which wearing bells does, leaves standing in aggression as the default Dothraki warrior's stance. But, it's the stance of a trapped animal, too.

The Stallion That Mounts the World prophecy is a desperate, even suicidal, ideal held up to embolden the Dothraki. On a meta level, it points out the fragility of the Dothraki way of life and insults the actions of those who would unite the world as conquerors, even as Aegon the conqueror did, by comparing conquest to survival by sacking.

That the dosh khaleen ever actually declared an unborn child to be the Stallion that Mounts the World suggests they have had some sort of vision, since it would make no sense to risk the confidence of the culture should that one child die in infancy. Yet, when Rhaego dies their prophecy has to be seen to have not come to pass.

At the same time, it is wonderfully appealing to see a narrative twist in the prophecy being true and applying to Drogon or to Daenerys herself. Assuming the books follow the HBO show in this, which of course they might not, we'd see Dany continue to live in the Dothraki way but from dragon back and with an eye towards conquering not just Westeros but Planetos. If so, I'd argue it would suggest it means she knows she has failed. Dany's stated dream has always been to go home, to end her life on the run. But Dothraki life is a life on the run. And the only outcome of any Stallion that Mounts the World is self-destruction, since the economies damaged by military conquest don't grow back as fast as grass does. Becoming the Stallion that Mounts the World would mean Dany truly has become Aerys come again, since she'd be taking out the whole world in her kamikaze mission to be the dragon.

I'd argue that while the dosh khaleen may have seen Drogon instead of Rhaego in their vision, that Drogon cannot be interpreted as fulfilling the Stallion that Mounts the World prophecy without a Dothraki rider such as Daenerys. So, the prophecy either applies to her and foreshadows her self-destruction or it's a red herring.