Indeed, WWI didn't start til August 1914... seems unlikely most of the 980K were killed in the war, more likely attrition/glue factory as they were replaced by cars/tractors.
I just read Storm of Steel -- which is, by the way, The Best First-hand Account of World War One -- and it does seem like you can't go two pages without running into a dead horse.
And from a numbers standpoint, if 6 million died in a 4-year war, it doesn't seem far-fetched that 980k would be killed in 6 months, especially since for an appreciable chunk of the war the Germans hunkered down behind the Siegfried line to conserve forces for a 2-front war.
The first few months of WWI weren't that intense and the full-scale horror was yet to become apparent. At this point it was 'just another war' rather than 'the war to end all wars'. Enough so that at Christmastime the opposing soldiers were giving each other gifts and in one case played a game of football. The battles of losing 20,000+ men in a single day were yet to come.
Actually, the war was every bit as intense in 1914 as it was in 1915-1918. 400,000 casualties at the Battle of the Frontiers. 250,000 casualties at the Battle of Tannenberg. Half a million casualties at the First Battle of the Marne -- which included losing 20,000+ men in a single day.
Remember -- the war in 1914 was a war of movement. Which meant that men were charging across open fields to attack the enemy. The Germans wore leather helmets, the other armies wore cloth caps, and the French wore red trousers. Yet the armies had modern rifles and artillery. This was a deadly combination.
Battles in 1915-1918 racked up higher total casualties, largely because they lasted longer. The Battle of the Somme lasted four months -- because they kept fighting over the same piece of land. Whereas in early 1914, the armies were still fighting a war of movement, so they'd fight in ten different towns, and the casualties would be spread out over ten different battles.
The Christmas Truce was not a question of numbers -- for a lot of men had already been killed or wounded -- but attitudes. The soldiers still felt that their opponents were fighting honorably. Poison gas, the British starvation blockade of Germany, unrestricted submarine warfare -- that would all come later.
if UK horse population went from 1m in 1900 to 20K in 1914, with half the decline being attributable to WWI in 1914, that would mean 490K out of 510K British horses alive at start of war died in those 5 months.
