Millions of horses and mules died in Wordl War I.
Steven Spielberg made a movie about the fate of the animals.
You can find information about the real story behind this here.
To make this photo must have taken lots and lots of preparation. Literally hundreds of men participated in the project. Look through your eyelashes at the mouth and eye of the depicted horse: from his high position Mister Raen must have given many intructions during the actual shoot too.
I did not study the background of it but I am quite sure that the men in the photo were volunteers. Many common men, unable to stop the most insane of all wars, not only cared about the human victims.