Centaur!

It was a kingdom of centaurs ruled by the Centaur King.
You, the son of the King and uniquely human, traveled east and west, far and wide across, in service of a land you were unlikely to ever rule.
Then one day, a message reached you on the battlefield. The King had declared a tournament of battle by horseback.
Whoever should win would have it all.

You can win. The time has come for you to choose a mount... a centaur who will let you ride her to glory...

A mythical novel ADV where all heroines are half-girl, half-horse!

[From DLsite English]