Here’s a post by Judith Tarr at tor.com about the big guys.
Modern legend would have it that he’s the descendant of the medieval destrier, but that celebrated war machine was probably more like the modern Andalusian or the Lipizzan (the latter with its substantial bone and strong build) or, though the breed itself came along quite late, the Friesian. What we know now as the draft horse is a product of selective breeding over the last handful of centuries, including breeding for size. The really, really big guys are a modern phenomenon. …
I always figure that when the apocalypse comes, horses will hang on. They’re too useful not to. Riding and lighter driving horses for faster-than-human transport over distance, and heavy horses to work the land. They’re two sides of the same important and historically valuable coin.
This is all well and good, but I primarily linked to it as an excuse to post the following picture, which is of a Friesian cross rather than one of the really, really big guys:
I have a poster of this horse — his name is Mystic Warrior, but he’s called Domino — anyway, I have a poster that shows him in a beautiful floating trot. Even though he is a horse and not a Cavalier King Charles Spaniel, I look at that poster every time I go down that hall, as a reminder of what a REALLY GREAT trot looks like.
Also, as you can see, he’s just a spectacularly beautiful horse. You really couldn’t put him in a novel, except cast as a semidivine horse like Shadowfax. He’s too unbelievable for fiction.