[William Cloud Hicklin]

Setting aside his "ROTC" stint with King Edward's Horse, Tolkien could hardly have been unaware of the ubiquitousness of horses and ponies, in and out of war! He grew up in an animal-powered world, and this persisted in Britain throughout his young adulthood (Britain did not automobilize nearly as quickly as the USA), and certainly in France he couldn't have ignored those of the half-million British Army horses there which he would have seen every day. And, yes, that meant seeing the poor beasts being killed.

As for "an example of equine courage" underlying the mare or Shadowfax- perhaps less likely. Tolkien would never have seen horses in combat,* and moreover cavalry's employment in battle is never, ever as "stalwart defense" (a horseman brought to a halt is a dead horseman).

*While cavalry was certainly used in WWI in its intended role in more open campaigns like Palestine, Mesopotamia and Russia, the 1914 Battle of the Frontiers quickly put an end to its use on the Western Front. Useless in trench warfare. The Germans transferred all of theirs to the east, or converted horse regiments to infantry, as did the British in many cases (my wife's great-uncle was killed with the 6th Dragoon Guards as a de facto infantryman). It is true that for the Somme the plan included a division of cavalry to exploit the breach in the German lines, but since no breach was made the cav remained in reserve, well back from the front)