>>715088060
Horse Racing is a young man's game. You have basically three years to leave your mark, and then you're done and it's up to the next generation. There is no true 'protagonist' in horse racing because of that. Symboli Rudolf was the protagonist of their generation. TM Opera O was the protagonist of a generation. Oguri Cap was the protagonist of a generation. Arguably, in sprinting, even Bakushin was a protagonist.
That being said, if you want the most popular Japanese horse ever? It's either Haru Urara (Who became so popular it went beyond horse racing) or Oguri Cap (brought the female gender into the sport through sheer charm appeal).
Opera made more money than both when you account for inflation, but that's only important if you're a betting man.