Verified answer

Perhaps part of the overture of William Tell by Rossini?

Galloping Horse Music

Learn Train Your Horse Easy!

Thought I may add one more name to the already extensive list: {not exactly unusual but illustrates the wanton slaughter and waste of war} George Butterworth,the dedicate of Vaughan-Williams London Symphony. A promising composer in his own right yet plagued throughout his short life by a sense of purposelessness. Perhaps this is why he spared little hesitation in enlisting as the great war engulfed Europe. Before leaving England for the trenches in 1915 he destroyed those compositions he deemed unworthy of survival. He was subsequently killed by a lone snipers bullet on 5th August 1916 at Pozieres as he gallantly lead a raid during the Battle of the Somme which over the course of some five months claimed the lives of over a million people.

Light Cavalry Overture, about 2 minutes into the piece./

here is the light cavalry overture.. what is most likely a horse song starts at 2:10

http://www.youtube.com/watch?v=XFQpPbjJWTc

and here is the william tell overture starting at 0:17

http://www.youtube.com/watch?v=c7O91GDWGPU

these are the only two likely pieces for horses