This month a British horse named Warrior, who served in the First World War, received a posthumous medal for gallantry. Awarded by the UK animal charity People’s Dispensary for Sick Animals (PDSA), the Dickin medal has been likened to the Victoria Cross, which is the highest decoration a human serving can receive. The award to Warrior was an honorary one, and was in part to recognise and commemorate the service of all the animals involved in the Great War.
Warrior was owned and ridden by General Jack Seely, and managed to survive on the Western Front from the date of his arrival, 11 August 1914, until the end of hostilities in 1918. He and Seely were fortunate enough to return home to the Isle of Wight, where Warrior lived until his death, at the ripe old age of 33.
Meanwhile, the only Australian horse to ever be returned home from service during World War One was General Bridges’ mount, Sandy. Something between 121,000  and 169,000 Walers  (as in, horse from ‘New South Wales’, as they were known, regardless of where in Australia they originated) were shipped overseas during the Great War. At war’s end, there were around 13,000 surviving animals, and it was deemed inefficient to send them back to Australia, given both quarantine concerns and expense.
Horses were to be redistributed to other armed forces or sold on wherever possible. The surviving animals were classified according to their age and soundness, with any that did not meet requirements to be destroyed (read: shot). The cut-off age before they were automatically consigned to destruction was 12 years for a riding horse and 15 for a draught horse, which is pretty awful when you consider that a horse’s natural lifespan is double, if not triple, that.
An article in the War Memorial’s magazine, Wartime, details the process:
After their manes and tails were shorn (horse hair was valuable) and their shoes removed, these horses were taken to selected spots near their camps where working parties under the command of a veterinary officer shot them with pistols. They were gutted and the skins salted (these were valuable too). 
It seems that 3,059 Australian horses ended their lives like this, shot by those they’d served and stripped of their hair, skin, and shoes for their value. I suppose when we consider the great number that died on battlefields, and the more-than-likely horrific manner of their death, this figure, and the clinical means by which it was carried out, pales in comparison.
There is a romantic idea that servicemen shot their own horses, rather than sell them to the Egyptians, who they feared would treat them cruelly, but it appears that this is nothing more than a myth. The truth is far more rational.
In 2009 the Australian War Memorial unveiled a sculptural work by Steven Holland, designed to commemorate the service of all animals in war.
When you consider how these animals were treated, it doesn’t seem like much, really.
 Jean Bou, “They shot the horses – didn’t they?”, Wartime 44 (2008) 54–57; available online at http://www.awm.gov.au/wartime/44/page54_bou/
 Jason McGregor, “And Only One Came Home”, Military History and Heritage Victoria Inc., http://mhhv.org.au/?p=830
 Bou, “They shot the horses”