Valuable horses were mostly transported in special horse cars, but Wikipedia says that the Keystone Company developed a more utilitarian car for transporting less valuable horses in freight trains, eventually producing some 1000 of them. I have not been able to find any other information about these cars (Google searches return the Wikipedia article) and hoped someone might have some more information. There were notable shipments of horses to Europe at the end of WWII (2000 through Portland, Maine, in late 1945- early 1946), but the Keystine cars were probably much earlier..