Felix Salten (1869-1945, pseud. Siegmund Salzmann) is infinitely better known for his other books; principally among them Bambi, the Disney film of which has introduced many a child to the concept of death . He was born in Hungary, but moved with his family to Vienna when very young as Vienna granted full ciltizenship to Jews. He lived most of his life there until forced to flee the Nazis in 1939. He lived in Zurich, Switzerland until his death in 1945. He was President of the Austrian branch of PEN (the English branch of which Josephine Pullein-Thompson was very active in much later).
He wrote several children’s stories about animals: the only horse-related one was Florian, Das Pferd des Kaisers, in 1933. This story about the trials and tribulations of a Lipizzaner who fell from glory to a cab horse after the First World War Schons felt reflected the situation of Austria - a once proud empire reduced to a small and poor country. Florian was translated into English very shortly after its publication.
Finding the book: The Engilsh Jonathan Cape edition is not easy to find: the J A Allen is easier but can be more expensive than the average pony book.