Dr. Yvonne Raden compiled an overview of the different German and English editions of “Gymnasium des Pferdes” / “Gymnasium of the Horse” that exist. You can find it here:
Overview of the editions since 1886
Gustav Steinbrecht (1808-1885)
Paul Ferdinand Pllinzner (1852-1920)
Hans Sigismund von Heydebreck ( 1866-1935)
If you struggle to follow the dialogues between Steinbrecht, Plinzner and von Heydebreck, here is the text for you: