Who knows? But one thing must be remembered. The U.S. Army Quatermaster Dept. was one of the best sources of supply for the Rebel. My assumption is that the Union Army mounted its troops on the best mounts available.
An excellent article on the mounts of the Civil War is War Horse! Union Cavalry Mounts, 1861-1865 by David J. Gerleman, in North & South Magazine..Vol.2 #2 January, 1999. If you are able, please obtain this article. It delves into the quality to mounts available to both sides during the war.
If you should not be able to find this article, let me know via e-mail and I will scan it and send it to you. This volume contains info on mounted troops on both sides.