The term "Buffalo" was used to describe white men in Eastern NC who joined the Union Army.
http://buffaloesoldiers27june1862.wordpress.com/2010/09/04/unionism-and-the-arcane-origin-of-buffalo/

In the case of black troops in the west the term Buffalo was used about 1866/1867 but in this case the term Buffalo was used during the Civil War and had a different origin.