A post I made about horses in another part of the forum:
"Most people who talk about cavalry, really know absolutely nothing about horses. Having grown up around them at my grandfather's farm, they're a lot more capable at doing things than what modern historians frequently say about them. The whole "horses won't charge into a solid object" is absolute bollocks. Horses can be trained to do these sorts of things. Horses spook easily with things they aren't familiar with. But you can train them out of this behaviour and get them familiarized with things. It's how horses have been trained to work with firearms, or how the horses for picadores have been trained to charge bulls in bullfighting and to have no fear of their horns or of getting gored (despite the fact that it happens very often).
In regards to whether or not cavalry can charge "solid" objects, I'd suggest reading Marcus Junkelmann's third volume of "Die Ritter Rome", where he explains you can train horses to run into, and bully into a line of formed men shouting and waving spears and swords and other weapons. The issue with horses not charging into a mass formation has nothing to do with the animal's nature, because you most assuredly can train this out of them. The issue is with the men riding the horses. Do they trust themselves and their horse and their long pointy weapon to be able to break the enemy's ranks and their long pointy weapons. Sometimes you don't even have to train the horse, they'll do it on their own:
http://www.animalplanet.com/tv-shows/untamed-uncut/videos/horse-charges-into-crowd.htm That horse would have without any coercion or force, trampled those people standing in a relatively tight group at around 1:05 if it hadn't tripped.
This link is also a good resource for studying the shock charge:
http://www.classicalfencing.com/articles/shock.php
Stressing this: "It is the resilience of that interface between lance and horse that determines the amount of energy put into the impact. Thus the amount of the horse's mass that could be effectively put behind the lance was directly determined by the rider's size and strength. In other words, a larger, stronger horse does not mean a harder hit, while a larger, stronger rider usually does. In fact, in a joust I am more concerned about a large rider on a small horse, than a small rider on a large horse. Velocity can increase or diminish the relative force of the impact. This is true both in terms of hitting and being hit."
Horses from even the best breeding stock are useless if they haven't been properly trained and blooded in battle. Chaucer writes that a destrier was 200 times more than a plough horse, and while some of that cost might be in relation to breeding stock, the training aspect of that horse is would also be factored in, and that training is far more important.
We can also look at the Strategikon where Maurice deems it very important that cavalry retain their formation, and that a cavalry charge ends up broken more by human psychology rather than animal psychology. Or even Tacitus and the Roxolani cavalry, where he alludes to the fact that not even infantry or cavalry could stand against them (having completely wiped out 2 auxilia cohorts in Lower Moesia), and only in poor conditions (heavy mud, combined with snow, when they were caught returning from a raid by third Gallic legion) were the Roxolani cavalry no match for infantry.
As for resources, Kelly DeVries is good, as is Anne Curry's book on Agincourt, and Andrew Ayton's "Knights and Warhorses". I'd stay away from Ann Hyland, who is rather biased and doesn't appear to have much knowledge of cavalry warfare.
So how vulnerable was infantry to cavalry? Pretty vulnerable if they cannot either convince the riders of those horses that making a charge is folly, or disrupt the momentum of the charge (most effective is through field works/prepared ground, and natural terrain features), or make the riders hesitate as they are committing to a charge. "